They're the best in-the-field counter to modern armor, which is the most powerful land unit in the game.
They are the best pillagers, as they get 4 moves. One tank and one gunship can pillage a town a turn and be relatively safe if you have a tech advantage.
They also can be good at mowing down large numbers of units. What I mean is that your advanced cavalry that have combat III can then go on to blitz. A gunship with blitz can kill up to 4 units per turn. One of my favorite great generals I ever had was a gunship with: Flanking I, Flanking II, Tactics (combined 85% withdrawal chance), leadership, all combat promos, and pinch. He was upgraded from a blitz/withdrawal cavalry warlord.
If you have a significant military tech advantage, you can have groups of gunships and tanks (to be able to take the city and not worry about SAM units), quickly take weaker cities as an auxillary stack before you can build regular stacks that can move 2 spaces per turn (with laser and robotics).
Disadvantages: Not amazingly high base strength for an end-game unit. Cannot capture cities or workers. Cities with only gunships will not consider themselves defended.
Overall I think they're a well-balanced unit. I only wish they came a little earlier, like with flight instead of advanced flight.
This would devalue the advanced flight tech, but to make up for that I'd make it so that jet fighters almost never lose to fighters, which would reflect reality.
As for paratroopers, they should be upgradeable to mech infantry, and they should be transportable with a bomber for extended range, where the interception of bomber and paratrooper occur independently.