Here's the basic way gates work - initially only key story gates like kvatch show up, then more start popping up across the map as you progress. Critical info is that you only need to close 3 specific ones to finish up: kvatch gate, bruma's great gate, and one other mandatory story gate. The rest are totally optional but good for grabbing sigil stones if you're into enchanting.

Once you wrap up that great gate quest near bruma, random gates stop appearing, though existing ones stick around until you deal with them. After completing the main story, all gates shut down permanently which is pretty handy for cleanup. Just keep in mind, if you're running any mods, they might change up how the gates behave.

Good to know - those random gates that pop up can be great spots for loot farming, especially if you're after specific sigil stones for gear enhancement. Just make sure you're ready for a fight when you head in.