Maybe I just didn't find worldgen settings, but I'd like to setup biomes size and frequency.

Normally that requires a dedicated mod that can change worldgen and every biome’s frequency and size. Not really something that can be implemented just in one mod that adds things to the worldgen pool, because there’s a lot of variables to deal with; which biomes often spawn near others, et cetera. I’d recommend terra for that, although I’m not sure if it’s exactly what you’re looking for. (edit: you can also change biome frequency/weight in the config files for terrablender)

Unless you’re saying that you’re using such a mod, and Gallosphere isn’t supported?

Galosphere does not support terrablender, this is very sad because when you put Terralith with galosphere, the underground biomes of galosphere does not generate at all. This there a way to fix this?

Normally that requires a dedicated mod that can change worldgen and every biome’s frequency and size. Not really something that can be implemented just in one mod that adds things to the worldgen pool, because there’s a lot of variables to deal with; which biomes often spawn near others, et cetera. I’d recommend terra for that, although I’m not sure if it’s exactly what you’re looking for. (edit: you can also change biome frequency/weight in the config files for terrablender)

Unless you’re saying that you’re using such a mod, and Gallosphere isn’t supported?