In this regard, can you level up guns in custom mutations?
No you can't, at least not with mutations on.
One may also ask, can you get achievements in custom mutations? But to answer your question no you can't do easter eggs in custom mutation games, nor do you get exp, nor are you able to get trophies or achievements. The custom mutations are basically only good for learning the maps.
Also question is, can you level up on custom games zombies?
Yes you can rank up in custom games. Yes you can. You can put it to round 20, and survive 1 round and you will get as much as you would survivng from round 1 to 20.
How do you get XP in Black Ops 4 zombies?
To farm XP in Black Ops 4: Zombies, you'll just need to jump online. Go to the standard “Zombies” and choose to add bots to your group. You can add up to three to your team. Bots will steal kills from you, but they make surviving the later rounds much easier solo.
