The AlphaTauri star will be joining an all-French F1 outfit next season, as he will be racing alongside compatriot Esteban Ocon.
Asked what convinced Alpine to pursue Gasly, Rossi explained that it all came down to performance and that they see great potential in the 26-year-old.
"Performance was the number one criteria. Today, our car is the fourth on the grid, which means in general we are aiming for 5th, 6th, 7th places in general," Rossi told the media.
"He was the fastest of the available drivers, or even the drivers who were not available.
"Pierre has demonstrated in the very recent past with a car that is not so good that he knows how to climb onto podiums and sometimes the highest step."
This comment alluded to Gasly shocking the sport in 2020 when he won the Italian Grand Prix at Monza with AlphaTauri.
Rossi added: "He is also young so he is the perfect candidate to help us grow, to grow himself, to help us grow with him and go higher than the 5th, 6th, 7th."