Reynolds even emerged in an effective race lead following his final pit stop on Lap 44, before immediately coming under pressure from van Gisbergen.
While he vigorously defended the initial attack, his failure to cover the inside line at Turn 9 half a lap later allowed the Red Bull driver to seize the lead.
“I honestly didn't expect to be thrown into the lead battle whatsoever,” Reynolds said.
“Leaving the pits I was a bit surprised to see him there because usually he just smokes everyone and you never see him again.
“We had a little battle down to (Turn) 4, I got a reasonable run out of (Turn) 7, went through Turn 8 half alright, looked in my mirror and I'm like, ‘I'm clear'.
“But cold tyres, I thought I braked at my normal point, maybe a fraction earlier, he just did a good job,” he added.
“It was a good move, but that's what happens when you don't believe in yourself. I never believed I could do a good job here.
“A part of me is kind of happy but also bitterly disappointed I didn't hold him off and try and win the race.
“If I jumped in my little time machine and went back to that corner, I would have blocked for sure, that's just the lesson I've got to learn.”
“If you said I was going to be fourth and second, and second in the championship coming out of this round, I would have taken it every day of the week,” Reynolds said.
“I'm very disappointed in myself to be honest, because once my tyres got up to temperature about a lap and a half later I'm like, ‘I'm actually faster than him.' That's the other thing I couldn't believe,” he added.
“I should be ecstatic with second but I'm actually kind of a little bit disappointed.”
At the flag, Reynolds crossed the line just 0.8s behind the race winner, and sits second in the championship hunt after the opening round.