Because that way Ray's name comes up most often around gambling dens, and since Sabaody is closely monitored area by marines they are more aware of what he is doing. Garp says the same thing.
That's the reason why his name still has so much clout, because people recognise him as the right hand of PK. Ray doesn't have a single individual achievement to his name as a pirate
Gaban has been practically non existent to the OP world outside of Elbaf for past 20 years. That's why his name doesn't come up often.