I dunno, it seems to me that a lot of Gem tech is gem activated only, plus while they got hella science they also have a bunch of magic going on as well. Unless humanity finds a way to data mine Gem computers I don't think they'd get much out of the actual stuff that was left laying around. Besides weapons, don't see why humanity didn't find all of this Gem weapons and be like, "Hey, let's turn this impossibly sharp weapons into bullets to fend off the Monsters that show up to wreck our families every once and awhile."