Albion came from a goal down to beat City 2-1 at the Amex on Sunday, with Gruda grabbing an 89th-minute winner.
The game turned on its head when head coach Fabian Hurzeler brought on the German and three other substitutes around the hour mark, as the hosts swarmed all over Pep Guardiola's team.
After the win, former Mainz star Gruda revealed what Hurzeler told them at half time, while talking up the quality of his team-mates.
"Yeah, we have to be more intense. We have to sprint harder. I think in the first half we didn't do it so well. If we sprint 100 per cent, chasing the ball and all together. It's going to be tough for everybody," he said.
"We are a very good team. Everybody who plays on the bench, we have fantastic players. And if we give 100 per cent and sprint, it's difficult for everyone, especially when the subs come in.
"He told us to sprint, to chase the ball, and we did it. And they had some problems."
Following Albion's first win of the new Premier League season, ex-City goalkeeper Joe Hart said Hurzeler's side exploited some ["alarming patterns" in Guardiola's team](https://www.theargus.co.uk/sport/25435810.brighton-spurs-exposed-man-citys-alarming-patterns/).
That included hurting them on the transition and exposing their high defensive line, while not being helped by having makeshift defenders out of position.
When asked if that was City's main weakness, Gruda replied, "If you don't give them time, if you get in their face and be assertive, you can actually penetrate them.
"I think it doesn't matter who we play against. When we chase them, when we press hard, when we press altogether high, it's difficult for everyone.
"It doesn't matter which team we play. And that's the key thing. We did this, and of course, as I said, there was a bit of luck."