Gas chromatography flame ionization detector (FID) - why hydrogen gas?
The key to the answer is understanding how FID works.

The hydrogen flame has a minimal flame ionisation, what is needed for the low signal baseline.

Incoming organic molecules from the HPGC column create in the flame a lot of ions and increase the flame electric conductivity.

Using alternatives causing higher ionisation would decrease FID sensitivity that is not great even at ideal conditions, compared to e.g. ECD.