My answer may be biased, as I despise Z-moves in Generation 7 but I think that Gems would not be a very healthy addition a 50% boost to a move is pretty insane even if they're one-time use, I will provide examples:so on an unrelated note, how do y’all think gems would impact the current metagame? who would benefit the most from them, and would they be balanced in OU?
+2 252 Atk Flying Gem Hawlucha Acrobatics (110 BP) vs. 0 HP / 0 Def Tapu Koko : 243-286 (86.4 - 101.7%) -- 12.5% chance to OHKO (A bit of chip and you get the KO)
+2 252+ Atk Flying Gem Hawlucha Acrobatics (110 BP) vs. 252 HP / 200 Def Clefable: 412-486 (104.5 - 123.3%) -- guaranteed OHKO
+2 252 Atk Ground Gem Garchomp Earthquake vs. 252 HP / 252+ Def Clefable: 430-507 (109.1 - 128.6%) -- guaranteed OHKO
+2 252 Dark Gem Atk Kartana Knock Off (97.5 BP) vs. 248 HP / 220 Def Zapdos: 367-433 (95.8 - 113%) -- 75% chance to OHKO
These are a few and it goes to show that providing a 50% boost to any move you want is ridiculously strong, it kind of allows you to just pick and choose what you absolutely nuke and it's a large reason why I hated Z-moves in Gen 7
Obviously they aren't the same as you can keep Z-moves until you see a situation fit while Gems are automatically used upon clicking a move of their typing while Z-moves are exclusive to one pokemon per team but I think they have the same principle in what they do
So TLDR I think that they'll cause a lot of Offensive pokemon to be a lot harder to check, Hawlucha gets a really big boost and Garchomp gets an option for better breaking capabilities without having to take recoil damage form Leftovers
Kartana is cursed etc.
They'd be fine though if they were 30%, a one-time Life Orb is not really that good if I'm being honest
That's my rant on Gems