[ncmncm]

There is no need to "ignite" the gas giant. You just skim off hydrogen and fuse it in magnetic confinement chambers anywhere you need power. Gravitational confinement is for amateurs.

We don't build bonfires in our cars, as used to be done in steam locomotives; we oxidize carbon differently, and generate a pressure differential by entirely different means. Why should advanced aliens do things the clumsy way?

Anyway there isn't enough material in a solar system to build one. If you can get elsewhere to gather the stuff, why bother to bring it back?