Wow , Nice ! :)

This stuff is all amazing!  

I hate to be 'that guy', but I've got to ask, why GPL and not one of the CC licenses?

Not trying to second guess your choice, however you want to share is fine with me. 
I actually ask because I have a long running project to update/re-write the submission guidelines and license FAQ, and the general take I've been going with for GPL licenses is 'these are designed for code, they are included on OGA to cover legacy cases, don't use unless you know exactly what you are doing'.  So I'm sort of checking if that's the right or if you have a different take on using GPL for art assets.

If you prefer sticking with your current one (GPL) that's fine, but I'll have to join capbros being 'that guy'; you may get a lot more people giving you credit overall with a CC-BY, CC-BY-SA, or OGA-BY license... Simply because more people may end up using your art to begin with.

Many people may have projects incompatible with GPL (and, as capbros said, GPL for artwork is a bit of a hexagonal-peg-pentagonal-hole situation), but CC-BY-* will work for your fans but still requires people give you full credit. :) You can also have multiple licenses. GPL and CC-BY-SA for example. People can choose the license that works best for them, but either way they must credit you and share all derivatives under the same license. :D

It's your call, though. Don't feel pressured to change licenses if you're not comfortable doing so.

I love your work.