Are GOG games already well packed? Or are they just stored?
 

Curious if its worth repacking GOG games?

no you can save space with alot of them, if not all.

GOG games - or to be more specific - their installers are inno setup scripts and are packed with inno lzma, so you willl get better output if you pack the game after installation with custom methods.

The only difference between GOG and f.e. Steam are the game binaries (and that GOG use installers for installation).