KiNgBrAdLeY7: If you enable your gog profile, someone searching it can see your games and percentage completed... Although, i think you have to play them through Galaxy client and/or allow cloud/sync.
FullOfTacos: Right, I was thinking more along the lines of a quick look up. More for a myself type thing so I don't have to search and count the games myself. I have just recently made the transition over to GOG as my full time client and thought it would be a nice feature instead of having to go through and count the 100%'s myself.
You can try to send a suggestion to customer support by selecting "Other issues">"Suggestions", or by opening a public wishlist dedicated to this function in the appropriate section.
They've always said they'll add new features to the profiles, so maybe sooner or later it'll be possible.