gas oil vs fuel oil

gas oil and fuel oil both are nouns.

Word	Noun	Adjective	Verb	Adverb
gas oil	Yes	No	No	No
fuel oil	Yes	No	No	No

As nouns, fuel oil is a hypernym of gas oil; that is, fuel oil is a word with a broader meaning than gas oil:

gas oil: an oil formed through distillation of petroleum of intermediate boiling range and viscosity
fuel oil: a petroleum product used for fuel

Other hypernyms of gas oil include heating oil, hydrocarbon.

gas oil (noun)	fuel oil (noun)
an oil formed through distillation of petroleum of intermediate boiling range and viscosity	a petroleum product used for fuel