The VG Resource

Okay, so now I realize that GGD doesn't just work for NDS, and can work for GBa. What should I use to get down to the image and palette files (if it even works like that for GBA)

Search. The file.
Usually they use 4 bits/pixel. Palettes should be 2 bits/pixel.
However, some graphics (usually tiles and less used sprites) may be compressed with GBA's internal L777 compression - you'd have to get a decompressor then.

You know, there's a topic for this. :/