Mining upgrades do not change gas harvesting.

There is an exception of sorts though, the Mining Foreman Link - Laser Optimization. It will decrease the cycle time on the gas harvester (or mining laser, or ice harvester). Harvester Capacitor Efficiency also will boost gas harvesters, though in most cases capacitor isn't an issue for gas miners (often being in either a large hold cruiser (cargo fit Exequror is quite nice), battlecruiser, or battleship). Mining Laser Field Enhancement will also boost gas harvesters though a percentage boost to a range of 1500 meters isn't much of a boost.

Of course, there is also the T2 version that takes slightly longer per cycle, but double the amount. Being able to fit 5x T2 harvesters is a significant boost over 4x t1 harvesters.

If you are at skill level 1, going to skill level 2 will let you have twice the number of harvesters. Doubling your rate right up there.