Since the latest update I decided to start playing Terraria again, but since I'd like to keep it fresh and the official updates only add so much, I was looking for mods. Apparently though most if not all significant ones are not compatible with the GOG version of the game? I would have loved to try nTerraria or Super Terraria World :(.

Also, might I just add that devs (in general, not just for Terraria) punishing GOG players because of pirates is ridiculous? Pirates have never been stopped from pirating just because a game was just on Steam. Every single dev that does this literally just punishes legit players only. Grr! Well, I'm not the first one to say this but I just really wanted to voice my disappointment from being stopped from modding the game anyway.

Any mods or places I can look for GOG compatible mods?

Are there any compatible texture packs that change the look of the character sprites in the game?

Sadly terraria devs, but mostly also terraria mod devs, are only focussing on that darn steam version. It is really annoying how we can't just join steam multiplayer games either.

I want GoG REVERSE-connect for terraria to get access to the STEAM version

Don't all the mods have manual installs? Isn't the GOG and Steam version the same number?
They should be compatible.

They are different code wise. The steam version has some steam specific code for integration into the steam client. It's minimal but makes binary patches built for the steam version incompatible with the GOG version. Your best shot is to install tModLoader (google it as GOG won't let me post links atm) and use tModLoader mods, of which there are plenty.

That's annoying. I always thought they were the same. So is it not possible to play with Steam players, then?

It is possible for Steam and GOG players to play together, as long as they're using the same patch and gameplay mods. The differences in the executables does not influence their ability to play multiplayer together, it only pertains to interfacing with Steam or not.