It’s Baldur gate 3. I own it on both platforms

I think i worked it out. Steam with verify so yes you can do this

I've never tried it; in theory, Steam has the ability to "repair" an installation -- which would mean it should be possible to start an install with Steam, stop it, copy the existing files there, and then let Steam repair them. If push comes to shove, one might have to mess around with some files to make Steam think that it has completed the download so you can ask it to verify the installation.

However, I've never even tried this, so I don't know the exact steps, what problems come up, and how to solve them.

I cut the game from Gog and pasted it in steam. Steam verified the files.

Originally posted by Kargor:

I've never tried it; in theory, Steam has the ability to "repair" an installation -- which would mean it should be possible to start an install with Steam, stop it, copy the existing files there, and then let Steam repair them. If push comes to shove, one might have to mess around with some files to make Steam think that it has completed the download so you can ask it to verify the installation.

However, I've never even tried this, so I don't know the exact steps, what problems come up, and how to solve them.

It’s still verifying what I did but I essentially just cut the folder from Gog and moved it to steam/common.

Will update on how it ends up.

Is it possible to add a non Steam Game to your Steam Library?
Games tab at the next to Help.
I think the non Steam game just needs to be installed on your windows drive, it works for me.

It all worked fine. Some mismatch data but it will give you a list. Just delete the files and steam will fix it