gas tank vs gasoline tank

gas tank and gasoline tank both are nouns.

Word	Noun	Adjective	Verb	Adverb
gas tank	Yes	No	No	No
gasoline tank	Yes	No	No	No

As nouns, gas tank and gasoline tank are synonyms defined as:

gas tank and gasoline tank: a tank for holding gasoline to supply a vehicle

Other synonyms of gas tank include petrol tank.

gas tank (noun)	gasoline tank (noun)
a tank for holding gasoline to supply a vehicle	a tank for holding gasoline to supply a vehicle