People seem to think that because gallia supposedly uses oil as a fuel, that means they're less technologically advanced than the other houses using fusion drives.
Chances are, gallia is using similar ship drives. Those ship drives require energy - electricity - to run. The difference is that the houses tend to use nuclear reactors (remember our lovely fuel called MOX?) to create that power, whereas gallic ships are most likely using oil combustion reactors to create that power. On the outside, the engines are pretty much the same, it's just what the core of the ship is using to power those engines that matters.
The houses are relatively rich in radioactive fuels, plutonium, uranium, and the like. Perhaps radioactive materials are scarce in Gallia, but that oil fuel is quite abundant, and provides an efficient and ready-at-hand resource to use as an alternative.