as a gunslinger that is not MM, can I relatively keep mobile?

yes but still as sniper you have advantage while in cover

as a gunslinger that is not MM, can I relatively keep mobile?

Not really as a DF, but as a Sabo? Sure. You'll have less DPS(less of Series of Shots), but you can be fully mobile. Especially with Snap Shot, Pandemonium and such utilities.

DF has a lot of channeled abilities I'm afraid. Cull and Series are a core part of discipline. Even if you were to replace Series with Snipes / autoattacks...you still have Cull. Which is a 3s Channel. And it's your heaviest hitter due to it forcing procs of DoTs. So...kinda?

Overall Snipers aren't very mobile at all. Marksmanship definitely has to stand still the most, then Virulence and Engineering needs to stand still the least, only Series of Shots is really mandatory, which should be used on cooldown, 3 seconds channel every 9 seconds. Apart from that the only cast you'll really be using is Orbital Strike every once in a while. That said, you'll want to maximize your time in cover either way, as it grants a nice 20% ranged defense chance as well as immunity to ability activation pushback and if you take the corresponding utilities (which are very solid utilities) you'll get ballistic dampers upon entering cover as well as a passive 5% damage reduction while in cover.

Generally when it comes to moving around while dealing damage Mercs and Sorcs have it better than Snipers.