Idk about Gaban to the same degree but I can say quite comfortably Rayleigh is no less than any old legend, including Garp, and Garp himself placed Ray in the same vein as taking on Whitebeard.
There will obviously be gaps. I don't actually believe they are equals. But yea given Ray's standing in this verse it's hard to see anything but a high-extreme difference here. Ray at his age has current Yonkou shitting their pants at his sight, Roger certainly wasn't a god tier that jig is up. There's only so much higher you can go than Prime Rayleigh, he's already clearly implied to be superior to atleast 1 current day Yonkou (aside from Buggy).
Roger doesn't need to be god tier. He was just the strongest with Whitebeard, a step above even yonkos. It's what their hype always suggested. So Rayleigh can have comparable portrayal to yonkos(Garp as well for the matter) while still being a step below Roger.
Illness is what stopped Roger. Even while Old, Whitebeard was known as the strongest, only being nerfed by Illness.
So while I certainly think they are implied to be relative, I still think there will be a decent gap with Roger / Whitebeard(high diff)
By the way with the road One Piece is taking, with Joyboy, Original Nika, Imu, First God, there will certainly be power inflation
