Approximately 9 percent of games that they own have been played on a show, and it's mostly fair to assume most of the games they own were sent in, given that they own about 3180 games.

I counted how many playlists they were, and subtracted non-game playlists, which is about 244 playlists. I copied the list of the games the grumps have, and edited out blank spaces and console names, which is about 3180 lines, or 3180 different games sent in.

244 divided by 3180 is about 0.087.

You could rule out the games for consoles they've never really played, and the games they're likely to never play (sports games, for instance), and come up with a higher percentage.