Just like Quake 1, if you already own Quake 2, the enhanced version is available as a free update. Although unlike Quake 1, Quake 2: Enhanced is also available on GOG day 1.
In addition to visual updates, there’s a new episode “Call of the Machine”, Quake 2 N64, a pass to the enemy AI which changes a few behaviors and attacks, removes machine gun recoil, a new infinite use item that shows the player where to go next, and a number of other qol and accessibility options.
Anecdotally, I find the addition of Q2 N64 very appealing. It’s a mish-mash of pared-down maps from vanilla Q2 and the expansions in a strictly linear fashion - no backtracking. Of course, there’s also changes to the lighting and a new OST by Aubrey Hodges. Being able to control the game from keyboard / mouse is a godsend. I played the game in emulator with a modern gamepad and it was barely, barely doable, even when I could manually set deadzones and sensitivities and the like. It was awful.
It’s linear in the sense that there’s only 1 correct way to go, but there’s so much back tracking and with no map and very little contextual clues where to go, it’s fair to look up how to progress once or twice if you’ve never played Q2 before.