Leclerc secured his maiden Grand Prix win in Formula 1, one day after Hubert was killed in a Formula 2 crash at Spa-Francorchamps.
Gasly says the build-up to the race was the most emotional one that he has ever endured, and that he was "happy" to secure two points on his return to Toro Rosso for Hubert.
"It was for sure the most emotional pre-race I ever had," Gasly admitted. "At 22, 23 years old, to lose one of your best mates... I've grown up with this guy since I was 11 in karting.
"We've been roommates, we've lived in the same apartment in the same room for six years, We've been classmates, I studied since I was 13 until 19 with him, the same professor.
"I'm still shocked and I don't realise how it can go so fast. It was terrible and I already plan to see all our friends. None of us really understand or realise what happened yesterday. Just super sad.
"Once you put the helmet on and go for the formation lap, you're in a different zone and different sate of mind. You don't think about it.
"But of course, straight after the chequered flag, it's the first thing that came back in my mind. I was happy to score the two points for him."
With Jules Bianchi losing his life in 2015 after he sustained severe injuries at the Japanese Grand Prix in 2014, Gasly says Hubert's death is more "terrible news" for French motorsport.
"I told Charles before the race 'Please win this race for Anthoine'. We started racing in the same year, Charles, Anthoine and myself," Gasly said.
"Anthoine won the French cup in 2005. We raced together for so many years and we all knew each other. Between Jules a couple of years ago, and now Anthoine, it's really terrible news for French motorsport.
"They were two great amazing characters, and it's really difficult to realise."