Garthed:  To salt & burn, gank, or put to rest a supernatural being or creature.

Supernatural has always employed the theatrical device of the "aside" to the audience.   As others have noted, the show is a Master of the Meta.  Chuck Shirley was an avatar for show runner & head writer Eric Kripke.  Damien & Barnes represented the many who attend conventions.  And love her, or hate her, Becky was something of a stand-in for fans.  
So who is Garth?  
I think Garth is a nod to all those people who imagine themselves riding around in the back of the Impala, offering ideas and information and helping Sam & Dean solve the case.  And, of course, along the way, becoming their friend and ally.  
I know many fans find Garth, and some of his idiosyncrasies to be completely unbelievable.  Sadly, I find them all rather familiar.
Take his drinking for example.  Yes, it is possible to get drunk on one beer!  My alcohol tolerance is a long standing joke to my friends and family.  I don't have any!  I get tipsy on 1/3 of a rum cooler.  I am slurring my words by the bottom of a small glass of wine. I am an exceedingly cheap drunk"¦. Which makes me wonder why I wasn't more popular in university.  
That leads me to another point.  You don't have to be raised as a hunter to develop good observational skills, or become skilled at reading people.  You can also hone those talents by riding the bench in sports, and being the one left to guard the drinks at the table, while everyone else is dancing.  It's not the most desirable position in the group, but it does allow for plenty of quality people-watching time.  You can form a lot of theories, and notice a lot of patterns by closing time! 
It probably explains why Garth figured there was a deeper level to the relationship between The Axeman and the young, somewhat inept janitor, who kept getting second chances.
Embracing what some may perceive as your weaknesses can sometimes turn them into strengths.  Could Dean have persuaded the little girl to tell him that she had a "grown-up drink" just before she saw her mother murdered?  Quite probably.  We have ample evidence he's the type of person children feel safe confiding in.  But, in this case, did he look bigger & scarier and perhaps more of an authority figure than the goofy guy with the sock puppet?  Most definitely.  So, if you were her age, and you knew you had done something you weren't supposed to do (albeit accidentally), would you talk to Mr. Fizzles or to Mr. Luscious? (Oops - For a minute there, I was thinking she was my age!)

Frankly, for me, the most unlikely, improbable thing Garth did the entire episode was to dig the grave of Jennie G, by himself!  Digging a hole that big, and that deep is really hard work, and requires mucho muscles.  I spent a summer digging holes like that.  I got very fit.
It's ironic this episode reminds me of that job, because that's the summer I met Dean Winchester.  Okay, not the real Dean Winchester.  But close.  This Not!Dean was a friend, and a colleague. He was tall, blonde, and muscular.  He was confident, charming and spoke with more than a hint of sarcasm and snark.  And as I discovered over the 4 months we lived and worked together, he bore many scars, both internal and external.  I'm convinced that Eric Kripke was hiding in the bushes, and when he saw my co-worker, the idea of The Dean popped into his head.
Of course, some of the other Supernatural writers were likely hiding in the bushes too, because that's when they saw a real-life Garth in action.  Not!Dean and I were assigned to go buy supplies for the work camp we were setting up.  I wanted to make a good first impression. I was determined to be a conscientious shopper, and make our meal allowance stretch as far as possible.  I was intent on comparing prices. I didn't notice what I was doing.  Imagine a young woman, carefully backing up into, and then tripping over a very large pyramid of tomato soup cans.
Cans went flying and rolling everywhere.  They took out shelves.  They took out other displays.  They nearly took out other shoppers.  A stunned silence consumed the supermarket.
Then, the sound of laughter.  Deep, hearty, from-the-gut laughter.  Not!Dean said: "I thought things like that only happened in movies.  I didn't think it actually happened to anyone in real life!!" Then he threw back his head and laughed some more. 
I definitely made an impression!  But, I also knew I was growing on him. Just like Sam and Dean developed a grudging affection for Garth.
It's possible there is nothing meta about Garth.  Perhaps the character was created simply to add some humour.  But I think he's a far more important story-telling tool.  Usually the boys are front and centre.  We focus our microscope on them.  But, with Garth they slide a little out of the frame.  Focusing our attention on him allows us to watch Sam & Dean through our peripheral vision. That's an interesting phenomenon.  Ever notice how some things become clearer when you're not looking directly at them? I liked this chance to see the boys from a sideways glance. 
Garth is everything the brothers are not.  They're big.  He's small.  They're exhausted, overwhelmed and grief-stricken. He's upbeat, optimistic and fun-loving.  Life has given the Winchesters pain, and the burden of being different.  Life seems to have given Garth the opportunity to use his quirky nature to his advantage.
Particularly poignant is how Sam & Dean react to Garth's straightforward suggestion that Bobby is "riding their wave" as a ghost.  Garth looks at the EMF coming from the flask and makes the logical deduction, for a hunter, that Bobby is now a spirit.  He is very neutral about it.  He even tells Dean about his own cousin, who was salted & burned, yet came back.  Intentionally or not, he's giving Dean a gift.  He's offering him the chance to believe Bobby is still with him, although in an altered form.  
Dean's hunter instincts, his heart and maybe even his own ears (judging by the momentary hesitation at the motel doorway at the end of "Party On") are telling him that although Bobby died, he never left.  But Dean is afraid to hope.  In his experience, the supernatural brings nothing of benefit, only more pain, more grief and more sacrifice.
As for Sam, he deals with his questions about Bobby's afterlife by researching away his hope.  He uses a Talking-board to try and contact Ghost!Bobby.  When it fails, Sam slams the door on any further belief in the idea.
Although they talk about it in "POG", Sam best explained the boys' predicament in "The Slice Girls.


DEAN: These papers just moved.
SAM: What? 
DEAN: I didn't touch them. 
SAM: (SAM gets out the EMF reader, which immediately starts to flash red and make noise.) It's all over the place.  Oh, and... power lines by the open window, where there's a breeze...  that could have moved the papers. 
DEAN:  Did you feel a breeze? 
SAM:  It doesn't matter, Dean. The readings are useless. 
DEAN:  Hey. Maybe, uh... (holds up his flask). 
SAM:  We burned him, Dean. 
DEAN: So what? 
SAM: So, what are you suggesting? 
DEAN: I don't know. What are you? 
SAM: Concentrate on something else. 
DEAN: Why? 
SAM: Because it's not Bobby! 
DEAN: Could be. 
SAM: No, it couldn't be. 
DEAN: Why not? 
SAM:  Because we want it to be. 
In "Party On, Garth", Garth was in the foreground, supposedly leading the way, while the brothers remained in the background, although willing to offer vital support when required.  However, it's significant to remember it was Garth who called them, and asked for their help.  He knows he needs their expertise and knowledge.  He knows he can learn from them.
But Sam and Dean could learn a few things from Garth too.  For instance, he seems to get some satisfaction from The Job.  That's something the Winchesters have lost over the years, probably due to the magnitude of the battles they've been fighting.  Garth also knows the value of simple pleasures, like laughing at a silly comic strip, eating brunch or soaking in a hot tub.  
Most importantly, Garth knows that giving someone a hug makes their whole day better.  Hugs spread love, friendship and hope.  ("I wuv hugs")
In that context, maybe being "Garthed" isn't such a bad thing.  In fact, the idea kind of grows on you.