I connected my gog and steam account together however GOG was unable to find any games compatible. I have over 400 games...

I'm assuming they would simply have to be games that is found both in gog and Steam?

You assume incorrectly, games on Connect are only available during a specific (relatively short) period of time.
Currently only Limbo is available I think, which you can see on the Connect page.

There's a mix of games that are/were on connect, plus the dates.