The last Gem was made in the 1960's I believe. Gem never had the marketing force behind it that Gillette did, and hence Gillette had most of the market share. I think their market share probably shrank until it was no longer profitable to continue to make the SE razors.
I think they are not made today, again due to popularity mostly. There are far more blade choices available on the DE side, and the blades are still widely available in many stores, so it makes sense that someone would make razors to hold the blades.