RE: What was the sin of Gomorrah?
Same as Sodom’s... the two are often mentioned as a PAIR.
See Matthew 10:15 and Jude 1:7 for instance.
And the other two towns were Zeboim and Adamah.
One could get rather tired of speaking of the sins of Sodom and Gomorrah and Zeboim and Adamah so it is easier to say simply “the sins of Sodom.”
There was a fifth town but God spared it because Lot went there, but he left not long after.