I've never been able to find reliable total sales figures for the 3DO, though I've seen everything from 1.38 million to 2 million.
But there have been non-pack-in games with a 50+% purchase rate among console owners -- I think one of them was a PC Engine CD game, though I can't remember what it was -- so maybe it's possible that Gex hit very high numbers.
Hmm, maybe. I considered that possibility (some guy from Id Software claims Wolfenstein 3D and Doom had a 1:1 attach rate on the Jaguar), but I found it hard to imagine that a game could find that percentage of appeal on a system with as broad a software library and user base as the 3DO. On second thought, though, the lack of competition in the platformer field could have played to Gex's advantage. The 3DO version of Soccer Kid preceded it, but that was just a port of a game which was already out on several platforms. I still feel like it's something of a stretch but I can at least see the reason in what you're saying.
I guess that might help explain the high production run, too; I can imagine someone saying, "Look, when this baby comes out, hundreds of thousands of platformer fans who had zero reason to own a 3DO up until now are going to be running out to buy the system. We have to be ready."