I'll throw in my two cents on it. I'm personally not a fan of it, conceptually and mechanically. In terms of just concept, it's another cheap in-universe gimmick that replaces the last generation's. It's so ridiculous in universe but not at all interesting. It's just "Haha big Pokemon go boom!" and that's... that's it. Nothing really interesting behind it, just "big". It's also a shame since the Gigantamax designs are great, but they could have just been Mega Evolutions rather than this gimmick which is going to get thrown out in the next game.
That being said even mechanically, I'm not a fan from what I can tell. There's a reason it got banned from OU, it was too much for a lot of Pokemon who could abuse it. The nice thing about Mega Evolutions was that, while some of them COULD be busted, yes, it also gave Pokemon who needed a buff one. Z-Moves, while I wasn't a fan of conceptually, was a once-per game move that a Pokemon could benefit from, but I'm not really sure if it was crazy busted since I don't know competitive Gen 7. I don't think it made any single Pokemon busted besides Eevee baton pass. But for three entire turns you can just make a Pokemon big and bulky and have three turns of screen nukes. The problem is that it's abuseable, I think. They should have just stuck with Megas and kept the mechanic that actually worked both conceptually, mechanically, and-not to mention-aesthetically.