Here, just aborting the mission straight away is enough.
As a side note, I tried to fix this by cleaning up the Gatling Laser HWP ruleset entry a bit and matching it to the HMG Cyberdisc's, since I didn't have any ammo problems with that. Didn't work, instead the HWP gained snap and aimed fire modes apparently because I removed the tuSnap: 0 and tuAimed: 0 lines. So instead of defaulting to 0/no snap or aimed modes, it's defaulting to some other values (maybe the vanilla OpenXcom Laser HWP ruleset entry?).