i wouldn't allow that you use both at the same time.
the primary purpose of a a sound suppressor is to reduce the sound by containing and limiting the amount of gas (and with that; sound) that escape the barrel when you pull the trigger (you also normally use subsonic ammunition which travel at reduced speeds / reduced kinetic energy - and game mechanic wise this would probably reduce damage and/or effective range but the smaller charge would possible also have a positive effect on recoil).
the purpose of a gas-vent, however, is to vent gases out as much gas as possible and even vent the gas at specific vectors - all this will reduce muzzle climb and in the end reduce recoil.
imo; the two are simply not compatible with each other.
the only benefit you would get with a sound suppressor operating at the same time as an integral gas vent would be reduced muzzle flash (which would possible make it harder to spot the shooter at distance - there are actual real life accessories that are called flash suppressor (or flash guard or muzzle break or...) designed to do this as well; they just don't exist in SR5 core) - but the actual sound reducing property would be mostly lost due to the simple fact how a gas vent recoil reducing system operate.
weapon with a sound suppressor in a gyro mount would be totally viable though
