The desktop environment is the interface of your system. Think of the differences between Windows 7, Vista, 8, 10, 11. That's the Desktop Environment (DE). The good thing on Linux is, that you have large choice about what to use. And you can even install multiple DEs at the same time and switch between them!
All choices are much lighter than Windows. There are differences, but everything will use less than 1GB of RAM, and the CPU usage is so low that it will not matter on a gaming PC.
The most popular options are Gnome and KDE. If you want a good experience, it's probably a good idea to stick to one of these two.
The default on Ubuntu, Pop!_OS, Fedora, and many other distributions. Polished and easy to use, but customization is a bit stiff. A solid choice, though not built for extensive customization. GNOME supports display unredirection, so you don't even have to mess with composition. Is fully-featured in X11 mode, but Wayland lacks VRR (Freesync/G-SYNC) support, so if you're used to using those features on Windows, you may want to use the X11 session or choose another option.
Looks much like Windows by default, and has many nice quality of life features, effects, themes, etc. Easy to customize, but susceptible to breakage as a result. Supports disabling composition. Though already lighter than Windows out of the box, KDE can be slimmed down dramatically by disabling effects.
Lightweight and customizable, but not as feature rich and user friendly as KDE. Supports disabling composition, but is inconsistent with doing so automatically.
Very lightweight, especially low on RAM, but discontinued.
Being the successor to LXDE, LXQt is also extremely lightweight.
The default DE on Linux Mint. Cinnamon attempts to be very simple and traditional, looking much like Windows. People often seem to report problems and development is rather slow and conservative. Disabling composition might be problematic.
Good looking DE. Does not support disabling composition!