-Two of the trainers in the Olivine Lighthouse are retconned into being Jasmine's gym trainers. Even though they... don't use Steel-types and don't battle you in the Gym. Why was this change made? It's weird and feels incredibly pointless.
-Blaine's gym in the Seafoam Islands is massively expanded and has a proper puzzle now. This one actually bugs me the least as there were trainers in Blaine's gym in Gen I and they clearly didn't just blink out of existence. So it makes sense that they'd have followed him there. Also, Blaine more than any other gym leader loves puzzles and the gym puzzle is organic and low-budget and actually really cool.
-Viridian Gym is completely remodelled and filled with trainers and now has a tile puzzle. This seems to have been done for the sake of a very forced Trick Room theme, which Blue's team isn't even optimised to fully take advantage of. This really feels like they rubbed the edge right off Blue's character, and it's lame. None of the trainers use particularly powerful Pokemon. Feels like it would have been far cooler to have had to complete a puzzle just to fight Blue, with no trainers on the way. Would have ratcheted up the tension.
Jasmine/Olivine Gym: Another little neat thing about the Olivine Gym is that originally Jasmine was a Rock-type specialist until her Onix evolved and she then switched to Steel-type. So instead of being covered in metal her Gym looked like a more simplified version of Pewter Gym, a stone cave with rocks all over.
If they weren't going to keep that aesthetic, as well for some reason add in "trainers" who you don't battle, instead of this metallic "garden" maybe lean into her being a Coordinator and change it to a performance stage. It'll be explained that though Jasmine is a world-class coordinator she's also very shy, so she made her Gym into a performance stage to help her overcome her stage fright. That way it explains why there would be people in her Gym even though they don't battle you; they're guests of Jasmine watching her performances.
Blaine/(New) Cinnabar Gym: I also approve of the change to Blaine's Gym in HGSS. In the original it feels sad Blaine is all alone, like why isn't the Pokemon League helping him? However in HGSS he has clearly converted the cave into a more manmade room with the colored tiles and lamps, and of course Gym trainers (I also wouldn't be surprised if the stalagmites were artificially places for the purpose of his puzzle but to keep the cave aesthetic). Clearly Blaine had some help, you'd probably think from the Pokemon League, to convert the cave into a more Gym-like room at least temporarily.
However Cinnabar Gym in LGPE is probably the best version of that Gym.
Blue/Viridian Gym: I never understood the Lego blocks, though at least him being the only challenger does fit with his character.
The change to the Gym in HGSS I'm not exactly against though. Now the spinner puzzle is annoying, though it calls back to the original Viridian Gym Puzzle when Giovanni was in charge. However Blue amped it to the nth degree, probably because he thinks its funny to see any challenge spinning around all over the place trying to reach hum. So I can see Blue doing that. As for the Trainers, well all of them are Ace Trainers so they could just be there to train. Sure, Blue isn't one to take on disciples, but at the same time he is running a Gym so if a trainer wants to stick around and train there, as long as they're not doing anything wrong, that is kind of their right and it may even be a Gym Leader's duty to watch over these trainer's training. Heck, maybe that's why in Gen VII it's revealed he moved on from being a Gym Leader to being the co-boss of the Battle Tree in Alola (though I wonder who's running the Viridian Gym now then...).