The half-time replacement helped turn the tide for the Bluebirds after a below-par first half before scoring his sixth of the season from close range to clinch the three points against their promotion rivals.
"This year we have a very similar sized squad to last year but there is more strength in depth and players go in and out on performance," said Mackay.
"It is about making sure you take your chances and grab it and Aron came in today and did that. He scored but also did exceptionally well in the second half.
"They are a great group with a lack of ego there and that makes them work hard for each other whether on the pitch, the bench or the squad."
Crystal Palace boss Ian Holloway was disappointed with his side's final 15 minutes, with Cardiff ending the stronger despite his Eagles chasing the game.
The visitors dominated the first half and created plenty of chances but once Gunnarsson nodded home what turned out to be the winner his side failed to test Cardiff shotstopper David Marshall.
"We started well and ended terribly," said Holloway. "I thought we stopped creating chances and in these games you can't do that and you have to keep going. It was as simple as that.
"You know with this crowd like it is Cardiff are going to come back and in the end I am bitterly disappointed in the last 15 minutes because if any side looked like they were going to score a goal it was Cardiff.
"After they scored I don't think my team did what they needed to do, in fact they did worse and not better. There was a lot about us that I was delighted with but I just didn't like the way we ended the game. We have got to look at that part of ourselves because that could be crucial come the end of the season."
- Sports & Recreation