He said on Goodbye Television Centre that he was watching the football in preparation for his first live MOTD presenting gig, and somebody came running into the room to tell Gary that Des was being evacuated from Aintree, they needed a host for Grandstand, and he was the only host around. He was left to fill numerous times while match reports were being prepared, eventually switching to an old snooker match, and was then told he'd get a "crash course" to do the videprinter. Safe to say, he probably wasn't worried about MOTD after that.
I'm sorry to keep on ruining everyone's stories by being a massive pedant but it wasn't Gary's first time presenting MOTD - he'd presented it a couple of times already that season, most notably in September 1996 when he presented it for an entire month while Des had a break after a frantic summer of sport. But it was the first time he'd presented Grandstand. In Bob Wilson's book, he says that MOTD and Sportsnight, especially in those days, were actually quite easy to do because they were very heavily scripted and there were lots of breaks where you weren't on, but Grandstand was a real challenge and "if you hadn't done your homework, the programme would find you out".