It's a demo yes but if you have the original Quake II then its a full game. You have to add the original Q2 data files. You also need an Nvidia RTX card to fully enjoy it as it was intended for RTX card owners but any card *should* work. After looking at the comments on Steam, it doesn't work that great using ray tracing. I'll give it a shot later on my RX 580
You can get the stand alone here: https://www.nvidia.com/en-gb/geforce/campaigns/quake-II-rtx/
edit: Just tried it, looks like you need a ray tracing gpu as it says but apparently it can be played with a 10xx series?. I wonder if that will rule out AMD ray tracing cards when they come (Navi?).
Steamer/GOG-er: ASUS Crosshair IV Formula | AMD Phenom II X6 1100T 3.7GHz all cores | Mushkin 8GB DDR3 RAM 1333 w/ 6-6-6-18 1T | Dual AMD Radeon HD 6850s in CrossFireX | X-Fi Titanium | Dual-boot Windows XP and Vista