You guys already know GaW has a bad matchup, so i'll just pointout some obvious points.
I do not know a lot about kirbys ATs, so i'll leave that to you guys, here is some basic GaW stuff.
Kirbys air game is good, but GaWs is better - His airials have more priority and range, however they MIGHT be slower than Kirbys, regardless they are better. However, sideb will come in handy, i am sure.
Also GaW is very mobile, and apart from the roll, spotdoge, and dash, GaW is faster than Kirby.
Off-stage he is hard to gimp, so dair and upb spikes (and others if you have any) will not work, unless the GaW is on high percent.
Kirby can do his little grab tactics on GaW, but im pretty sure GaW can interrupt them with a fair or bair, and because he is light i think he gets sent further away then most other characters, which might be a problem for chaining together airials, i dont know really >.<
Kirbys downb will be useful against GaW, as GaWs will use upb to punish a falling character sometimes, and kirbys downb can combat this. It is also a good killing move, and kills upwards, so I really suggest you try to set this move up at high percentage against a GaW so that you can kill him easily.
Kirbys upb can be bucketed by GaW, keep this in mind, and remember GaW cant bucket-brake with a full bucket, so maybe you can use this to your advantage. You could also use upb to fill the bucket and then punish the end-lag of the bucket.
Pros of this match-up
GaW is light, he can be killed upwards easily with UpB or Usmash
GaWs roll and spotdodge are bad, if you space well or think fast it should not be hard to grab him
On the ground kirby is faster than GaW. Worry about SH Bairs, though.
GaW, who is good at gimping, will have a little more trouble with kirby.
Kirby has good options to get off the ledge Vs. GaW, compared to others.
Cons of this match-up
Kirby is light, and GaW has powerful smash moves
GaW can bucket-break, however this does not work as well upwards (so try to kill him that way)
GaW can bucket upb. Do not let him fill this bucket unless you are having a really hard time killing him and can afford to do this. You could make him bucket by using Upb and then just punish that straight off, too.
Kirbys air moves do not outrange GaWs, and GaW is better in the air
GaW can techchase into smashmoves, i said that because well, its a pretty common tactic and it can cost the match.
GaW wont be easy to gimp, and also does better off-stage than kirby.
Dtilt will get annoying. It can cancel your Fsmash, and well, i dont think you will like it very much.
I wrote more cons for GaW than pros for kirby only because i dont of the cool things kirby can do.
i hope this is a good start, pretty much everything is right/not very wrong. some of the ideas in the first section might not be good tactics though.
Yeah. just wanted to try to help.
I like matchup thread discussion.
Also i assume you all know what bucket braking is.