I'm not completely sure what gaming features it has, but I think it's supposed to spend less resources while you are gaming, I think, or something like that, but you know what else is great for that? Not opening the browser lol
The standard Opera browser was redesigned and rebranded as Opera One on version 100. It brought some new features including this grouping feature which is probably the only new feature beside the redesign that you will notice at first and probably the only new feature you will ever use in a daily basis. BTW, it may not show the Speed Dial button by default, but you can add it back in the settings
Try it out if you want. Just don't uninstall the GX variant yet, in case you don't get used to One.