The Brazilian started the midweek win over Shakhtar, scoring one and producing a superb all-round display.
“It’s difficult to find anywhere in the world a striker like Gabriel,” Guardiola said.
“The number nine for Brazil, his behaviour in training is outstanding and he has to fight with one of the best strikers I have ever seen in my life in Aguero [for a place in the side].
“I will not find a player like Gabriel. To sub Sergio you have to have a top player.
“I am so delighted with the strikers. When Gabriel plays, he does absolutely everything. Like he did in Ukraine, not just for the goal, he fights and plays.”
City face Watford this weekend, a game that sees the Club celebrate its 125th anniversary.
The last time we faced the Hornets was in the FA Cup final in May, with City producing a ruthless display to win 6-0 and become the first side in English football history to win all four domestic honours.
Since then, Watford have sacked Javi Gracia and brought Quique Sanchez Flores back to the club for a second stint as manager, and they looked reinvigorated during last weekend's 2-2 draw with Arsenal.
Guardiola says our dominant performance in the final at Wembley will have no bearing on Saturday's game.
"It was the last game of the season; we played that game knowing we could be part of something unique in English football history," Guardiola said.
"We had a few problems in the beginning but after that it was good. We won the Premier League a week before.
"Now it’s completely different. It’s the beginning of season, they have Quique Sanchez Flores. I saw the second half against Arsenal and, wow, it was incredible how they play.
"Every game has problems. I don’t compare with FA Cup game."