Ok, since Fables only posts PR Good News™ and doesn't answer questions, I did some research to answer
my question myself.
The installers with Galaxy included have _(g) added to their filenames, so for Witcher 3 GOTY:
Classic installer
setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709).exe
Galaxy installer
setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709)_(g).exe
If you download backups through Galaxy you seem to get the classic installers. At least that was the case for the few test downloads I've made. The only way to actually tell is to look at the filenames and check if they have the _(g) or not.