The bestsellers list on the GOG.com store page shows which games you already own among the current weeks bestsellers.
Finally, I have reached my goal.

So close. :P

5/5 ;(. Well, it's not that sad, I won't buy those 5 because I own retail copies.
Pierożek.

I have all but one...

I've got four from here. Four retail. One of the others I'm somewhat interested in for nostalgia, but that's it. Anyway, that's not bad.
Sanitarium is recommended if you're at all into adventures or darkly surreal games in general. Definitely a true GOG, that one. They've also done a bloody good job of fixing it up, too.

To be honest, the version I played was so awful that it's hard to tell how hard it was - it was made all the more difficult by dialogue boxes lasting fractions of a second and audio being largely absent for most sections of the game.
Like I said, the GOG guys really worked miracles on it. It works perfectly now. But, the fact I was willing to endure those shortcomings speaks volumes for the game's appeal.

I see the same here. My wallet weeps.

i own retail of psychonauts and w/o extras I'm not interested.
i also am not interested in homm3.

One missing.

Yes, I currently own four copies of Psychonauts (all of them beaten), through sales or forgetting I owned it. I'm hesitant to pick it up a fifth time, even if it's from an excellent website such as this.

Have all the top sellers. Psychonauts I have elsewhere and I can't justify a second copy right now, so the list is nearly all green for me.