Do you immediately understand what GemOn is?
Yes. But I might not have understood what you wanted me to understand.
Is it easy to navigate and start playing something?
I did not try the external site.
Does the idea of discovering games this way interest you?
No.
Would you use something like this to discover games?
No.
What feels confusing, unnecessary, or missing?
The scope.
From a concept perspective, you want to make a playable trailershow. That will not work for many games on a technical level. Especially not for regular jam games. Creating the mini demo version would probably be the same work as creating the game itself for a lot of jam games. Do not forget the bug fixing.
Also, a lot of demo versions would just not work to show off the game in 5 minutes. Like, how do you do that for a visual novel? Sure, you can make a trailer, but reading the intro will maybe just not be interesting. One could just as well look at a well made project page. The project page itself is the thing that should present the game idea in a way that makes the game interesting. I think puzzle games and action games might be best suited for the approach. Other games might be better presented with a regular video trailer.
Binge scrolling actual small games, or dedicated mini demos might work. But the games and or demos should be held to a curated minimum standard. Else it will be just spam or sloppily submitted whole games, instead of specifically made small demo versions or jam versions.
(Whatever you do, you also want it to be better than just using https://itch.io/randomizer )