"I have Baldur's Gate Enhanced Edition on Steam and Gog but Gog Galaxy only lets me install from Steam"

This, literally this, the hell do I have to do to get GoG Galaxy to install the stuff I have on GoG instead of their competitor?

Like I have the Enhanced version on Steam and Gog, but I have the DLC on GoG, but GoG Galaxy isn't giving me any options to download for GoG instead of Steam, what gives?

Don’t use Galaxy, you will avoid many problems that way.

If you are on Windows, you should instead use the so-called "offline backup game installers". These are the real DRM-free installers, (mostly) without Galaxy ties.

in your library select the game and right click to context menu. The most bottom selection with rhombus icon (as it is at the moment) will be selection of what exact version you want to play.

The similar icon can be found on a game page in the top row.