[DaiPlusPlus]

The energy required to dismantle and fuse a gas giant would be tremendous - there's definitely an opportunity-cost involved.

Compared to a Dyson sphere - which only requires a tiny, tiny, tiny fraction of the energy absorbed to build and launch the orbiting platforms that make-up the sphere.

You can "ignite" a gas-giant by maneuvering a black-hole into position inside the gas-giant, but (as far as we know) there are far more gas-giants than there are black-holes, and the distances involved are tremendous (nearest black hole: 3,000ly, nearest gas-giant: 6 to 9e-5 light-years). It just doesn't strike me as feasible or even worthwhile even if it was feasible.

Maybe as the heat-death of the universe approaches, the last band of remaining humans (don't ask) try to ignite the gas-giants to keep the lights on?

[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?