Black Widow from a few years ago would have won. She is a human and as such she ages and has actually ages a lot of years in the MCU.
In the comics she had a super formula but this MCU version is weaker and was already losing to an alien.
Gamora is an alien and seems to be faster and stronger and even more skilled in training.
First of all, You need to have some Black Widow Tactics if your going to fight the daughter of Thanos. They both know hand to hand and knife/blade combats so it wouldn't be slaughter but someone would have to lose in the end and I really do not know who would win. All I know is that Black Widow would lose if Nebula intervened and took her sis's side in the fight.