I have a game (Omerta) on Steam, and some DLC just went on sale on GOG. If I get the DLC on GOG, will there be any problem patching it to a game originally installed on the Steam platform?

I figure there will be no conflict, but I wanted to check before I spent my money. Thanks!

No idea. It depends on how GOG checks for the base game's presence, and if the game differs for each vendor. It is quite likely that GOG's installer is checking for the GOGGAMEOMERTA registry entries, which steam's version doesn't have, thus claims not to find the game. Unpacking the installer and copying the files manually may work, but it is possible that you will also have to add any relevant registry entries.

Add to it the fact that steam may decide to do an integrity check and/or update, and replace any modified files, and it is quite possible that you will run into trouble.

That's a pretty interesting conundrum there. GOG games are addable to Steam, but DLC sounds like it'd be a complicated pain in the butt.

Hmm. Thanks for the replies, guys. Seems like it would be a bad idea to try. Guess I'll just wait for a Steam sale instead :(