Tim Nevits has posted some time ago on the FB Gemini page a simple yet effective way to fix the problem with the space bar. Hopefully this won't be necessary anymore with the new mats being sent to existing owners and mounted by default on all the next devices, but just in case, here are the instructions and a few photos (reposted with permission from Tim):
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Finally hacked my space bar to respond when hit off center - with a tiny piece of paper no less! My Gemini's Achilles heel be gone! I'm using it as a "spacer" between the rubber ring under the bar and the key itself. The peg from the space bar goes through the hole in the paper. I figured when the key was hit on the side, the rubber ring wasn't getting pushed down far enough. The paper can't be too thick, or it won't work (a normal 70g/m2 A4 sheet of paper will do). There’s not much guidance needed. It takes a bit of fidgeting but it's really simple."
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Varti