The experienced ex-France international felt his team gave up the ball too cheaply during a demanding encounter in regional New South Wales
The French veteran had a gruelling introduction to football in Australia, sweltering through a 30-minute cameo off the bench at home to Adelaide United in November.
On Sunday the former Arsenal and Chelsea defender completed the entire game in 40-degree heat at the Lavington Sports Ground.
Speaking to Fox Sports just after the final whistle, the 36-year-old described the experience as being even more demanding than on his debut at nib Stadium.
"It was worse," he said.
"Very difficult to play. You know, it was too hot. It's really difficult to give our best. It's also difficult for the fans to stay here for 90 minutes. We have to be careful. We have to think about the health of the players. That is very important.
"That is not an excuse because today, we lost. It's not only me. I think all the players, it was quite difficult for them to play."
Gallas felt the defeat was a consequence of Perth's inability to control possession against the in-form Heart, as they surrendered all three points despite getting back on level terms mid-way through the second half.
"Yeah, I was disappointed because we (didn't) control the ball, that is our problem," he said when asked if the Glory could have gone on to draw or even win the contest.
"We have to play better, we have to keep the ball because today, for example, it was real difficult to play 100 percent. It was very hot so you have to control the ball, you have to keep it. But we didn't do it. And we were punished."