Just zip up the PAK file you generated for Steam. It should be located in Documents\Larian Studios\Divinity Original Sin\Mods

For the Nexus you'll have to create an account there.

GOG doesn't host mods.

My D:OS Mods:
Dunamis - A 15 hour Adventure Mod for D:OS EE

A Necromancer's Crusade - 10 hour adventure mod for D:OS EE

4 Character Creation (EE)