He didn't do it on his first try at THAT place. It doesn't say it was his first try ever. He, like all gunters, played all sorts of 80s games so he could be as best prepared for the hunt and challenges as possible. There was probably a reference to Pacman in the Manuscript, after all, it is legendary game. It made sense for every gunter to play and get good at as many 80s games as they could get their hands on. He never played a perfect game, but he played Pacman before. It is his knack. Some gunters are better researchers, Wade is 80s game player.
Here's the quote on First Gate:
“And you just walk in here and nail it on the first try!” She started
pounding her fist into the center of her forehead, and I realized she
was pissed at herself, not me. “Listen,” I said. “It really was luck.
I’ve got a knack for classic arcade games. That’s my specialty.” I
shrugged. “Stop hitting yourself like Rain Man, OK?” She stopped and
stared me. After a few seconds, she let out a long sigh. “Why couldn’t
it be Centipede? Or Ms. Pac-Man? Or BurgerTime? Then I’d probably have
already cleared the First Gate by now!” “Well, I don’t know about
that,” I said.
I had several false starts. I would play for an hour, or even two;
then I’d make one tiny mistake and I’d have to reboot the machine and
start all over. But I was now on my eighth attempt, and I’d been
playing for six hours straight. I was rockin’ like Dokken. This game
had been Iceman perfect so far. Two-hundred and fifty-five screens in
and I still hadn’t made a single mistake. I’d managed to nail all four
ghosts with every single power pill (until the eighteenth maze, when
they stop turning blue altogether), and I’d snagged every bonus fruit,
bird, bell, and key that had appeared, without dying once.
He has a knack for video games, he played Pacman before, he extensively researched Pacman and it took him 8 tries at that place before he played the perfect game. And that perfect game took 6 hours; previous 7 tries took between two hours and 6. Which means he was trying to win it seriously for almost whole 24 hour day!