Just make it easy on yourself: start with using a Fedora LiveUSB just to try out the different desktops.
Gnome is more like MacOS KDE is more like Windows
I'd just try those out first until you're more familiar with things. All distros are pretty much the same as far as features go under the hood, and you won't see a massive gaming difference on any of them.
Sticking with AMD hardware is the right move if you're gaming. Steer clear of unbranded machines you can't easily check compatibility on as you may end up with unknown hardware without proper driver compatibility. Some top gaming brands that make an effort to be Linux compatible:
- MSI
- Framework
- System76
- Dell (only the enterprise stuff)
You'll probably only find good deals on the MSI stuff in the upcoming sales, but maybe the Framework refurbished store online will have some desktops. System76 rarely has sales, and are a bit overpriced in IMO.
If you build your own, just stick with MSI and AMD. You can probably build a solid gaming rig for $750-ish.