The real joy of GEM is going back through the levels to get those high ranks. I bet most people just played through to the end of the game (if they made it that far to begin with) and called it a day without ever intentionally attempting to nab some thrilling perfect clears, which is a shame. I also think the 77/100 Metascore is representative of many critics judging the game without allowing it the proper time to sink its claws in, because I think it deserves much higher. Personally, I'd give it an 85+.
Both of them run fine, as expected from M2. Personally, though, they're the most skippable of all the SEGA AGES titles released so far.
Gain Ground benefits from the TATE mode support that the Switch provides, true, but while I was playing it I couldn't stop thinking about how much a 3D Classic iteration would drastically improve visibility of projectiles and add in much-needed enhanced depth perception when enemies are attacking and throwing stuff. Also it's just such a chore to play when your character is moving slower than molasses and has no hope of dodging the deluge of bullets, spears, etc. that become more and more unreasonably plentiful the farther you go. M2 could've easily added in an optional increased running speed toggle to help balance the difficulty a bit, so it's a shame that one doesn't exist here.
Alex Kidd just flat-out sucks. I have no prior experience with Miracle World since this year was my first crack at it, and it disappoints me to say that it hasn't held up at all. After reading all of the glowing praise for this online, I think there's a huge bias from the legion of '80s kids who grew up with it as the default built-in game that came bundled with millions of Master Systems, so the nostalgia haze is strong with this one indeed. The controls are too slippery and temperamental, the level design is random and nonsensical, the music is overly repetitive, and it features some of the lamest boss fights I've ever encountered. Rock-paper-scissors? Seriously? Maybe the other Alex Kidd games are marginally better, but going off of this first installment I see now why SEGA rarely pays tribute to this series and why there's no movement among SEGA fans to bring him back after a nearly 30-year hiatus.