I haven't listened to his latest, so am not sure what he's referring to. Right now 1-4 are are Sinner, Zverev, Alcaraz, and Djokovic. One of those guys is an outlier, and I think there are several other players with similar or better chances as Zverev--Medvedev, certainly, and maybe several others are close.
That said, Sinner and Alcaraz have really separated themselves from the pack. Right now Novak is the only one that I could conceivably see beating them for a Slam title, though the further we get from his last, the less likely it seems. I suppose someone like Hurkacz could catch fire at Wimbledon and do a "Cilic 2014." I think Zverev and Tsitsipas have the talent to challenge, but clearly not the mental game. Which leaves the younger guys, starting with Rune. He's got the talent to at least be third fiddle to Sincaraz, but the jury is still out on whether he ever actualizes it.
But I think the point is that, for the foreseeable future, the Slams look like Sincaraz's to lose. Surprises can and (eventually) will happen. Maybe one is hurt and the other gets upset, and someone else takes home a title. But there's a real possibility that we're at the beginning of an era not that different than Fedal in 2005-10. After Safin won the 2005 AO, Fedal won the next 10 Slams, 17 of 18, and 21 of 23 (through 2010). I'm not sure Sincaraz will be that dominant, but right now it doesn't seem impossible, maybe not even unlikely. But it is hard to assess the younger guys - the Mensiks and Fonsecas.
So my guess is that, without listening to Gill's latest, he's pointing out that as the field stands right now, we're in a Sincaraz era, with really only Novak being a serious contender--and maybe Medvedev, though I'm not sure he can beat the Big Two anymore on the big stage.