EDIT 8/24: I've updated the build and touched up the description to match! I've been running this in Steel Path solo mission no problem, except for Excavation, I just can't seem to play that game mode solo =/
The ONLY game mode I don't recommend using Gloom is Defence with a group. You'll simply slow down and piss off your teammates!
Quick and Dirty DPS Build
One thing to note: This build is EXPENSIVE. 197,780 Endo is nothing to sneeze at.
You can find my Beginners [Saryn] Build here
And my Intermediate [Saryn] Build here
Exilus: There are many compelling options here IMO. For just starting I recommend [Handspring], as nothing will kill you faster than being flat on your back while the enemies are shooting you. [Cunning Drift] if you want +15% Range(I don't think many of the "Drift" mods are worth slotting as they give you a marginal increase to the stat but you lose out on the utility of other choices). [Mobilize] is a great option for 5 points, giving you 20% increased Bullet Jump power and Aim Glide duration.
Giving [Saryn] Breach Surge from [Wisp] is something I've had good results with. The blind will buff your damage even more, give you double procs on the spore damage and help cc the enemies. The radiation procs from Breach Surge will also add another layer of cc, although really not all that important.
Giving her Roar is absolutely an option, you will slaughter crowds faster, but I prefer Breach Surge as it adds some CC to her kit.
Pillage is a compelling option for quick shield gate resets.
Once you're done maxing this build, check out my personal Build with Umbral and Aura forma included! I, personally, think it's [Saryn]'s Final Form!