bansama: No it isn't. For one thing, continuing to display the products is helpful given the fact that those people can still gift games. Thus, a constant reminder of games that are available can help when it comes time for such people to gift games.
Further to that, other people may be viewing the PC/whatever device they are browsing from at the same time and can then see a selection of games on offer. If they just saw an empty space, they'll get the impression that the site has nothing to sell. That's bad for GOG's image.
The very basic point here though, if it ain't broken, don't fix it. And the display of a few games you may already own, isn't "broken".

Navagon: Because obviously my post was deadly serious. :P

bansama: I know, but it's best to explain these things for others so they understand why it wouldn't be a good idea. For the record, Steam used to something similar -- until it broke the store for people who had purchased everything.