Are all Oblivion gates closed?

Yup, once you complete the Main Quest all of the oblivion gates close. Though, that doesn't include the gate to SI or the gates associated with Peryite's and Boethia's quests.

Thanks, just so I don't create another topic; Would killing all non essential NCP's in the game make it a bit of a ghost world or will they respawn?

Nonessential NPCs that have an actual name will not respawn (with a tiny number of exceptions), creating a pretty empty world.

Generic NPCs like 'Xxxx city guard', Imperial Legion soldier/rider/forester, Courier, along with generic baddies of course (bandits, necromancers etc) will all respawn after 3 days.

In general, if they have a name instead of a generic description like "guard", then there's only one, and no respawn. Many characters become temporarily essential when they are involved in a quest, and are otherwise killable, so you can be locking out quests if you kill them too soon, but if you do so after all their quests are done, there's no problem. Alga, the bard in Bruma, is unusual in respawning although she's a named character.

It would be a bit of a ghost world, with a strange balance to the population. Regulus Terentius is the only non-essential Count, for example, but IIRC all the beggars can be killed.