The ship cannot function properly without a captain or an enigneer
A ship can't function without at least TWO engineers.
is it too much to ask to do the same with the most fun role of GOIO, shooting?
...did you know that engineers can also shoot guns? And in fact, generally can shoot MORE, because they can keep their guns repaired effectively and not on fire without relying on an engineer to extinguish it for you?
A gunner doesn't necessarily shoot more. A gunner just is less flexible in terms of being able to repair. Extra ammo types aren't really that important. The vast majority of guns have one single ideal ammo type (or at least ideal for the build and circumstance).
I don't know who originally said it, but there's a quote: "Gunner is a binary class, 1 or 0," meaning that a ship works effectively with either 1 or 0 gunners. Having an engineer loadout doesn't change your role at all -- you're still there to shoot the dickens out of the enemies and out-damage them.
Some guns, like lumberjack, heavy flak, and mine launcher, really do benefit from having three ammo types to choose from. Hwacha and hades also to some extent, but it depends on the map, build, and pilot -- sometimes just one extra ammo as an engi will suffice.
Not being gunner CLASS doesn't mean you are on guns any less. It just means your loadout is more flexible and you're more independent and don't have to rely on an engi to keep your guns chem sprayed. It also potentially frees you up to have a buff hammer along with pipe wrench and extinguisher/chem so you can buff your own gun and deal additional damage.