How he ranks against other #9s when they were his age (going off Gabriel's 2017-18 season when he turned 21; ranked top to bottom by goals per game):
Harry Kane, second full season at Spurs (after a number of loans), 31 goals in 51 games (0.61 goals per game)
Karim Benzema, fourth season at Lyon, 31 goals in 52 games (0.6 goals per game)
Roberto Firmino, fourth season at Hoffenheim, 22 goals in 37 games (0.59 goals per game)
Mauro Icardi, second season at Inter, 27 goals in 48 games (0.56 goals per game)
Gonzalo Higuain, third season at Madrid, 24 goals in 44 games (0.55 goals per game)
Luis Suarez, just signed for Ajax, 22 goals in 44 games (0.5 goals per game)
Kun Agüero, third season at Atléti, 21 goals in 47 games (0.44 goals per game)
Gabriel Jesus, second full season at City, 17 goals in 42 games (0.41 goals per game)
Robert Lewandowski, first season at Poznań, 20 goals in 48 games (0.41 goals per game)
Romelu Lukaku, first seasno at Everton, 20 goals in 48 games (0.41 goals per game)
Edinson Cavani, first full season at Palermo, 5 goals in 37 games (0.14 goals per game)
However...
How many of them were playing for a team challenging for titles in a top league against packed defences every week? Gabriel has less space and more pressure than many of those others players will have had at the same age. Other than Higuain? Nobody else.
And i doubt many of them had the quality of movement that Gabriel has now. Suarez maybe?
And i'd bet every one of those players was getting regular starts, whereas Gabriel comes on as a sub in a lot of games, then he'll start one and not start for three games...