You're out of your mind if you believe that either Tony Parker or Manu Ginobili will end his career with the Spurs as a better player than George Gervin. They're both very good players and each is legitimately an all-star in his own right. But there's a significant difference between being very good and being great.
George Gervin was a First Team All-NBA player for 5 consecutive seasons and a Second Team All-NBA player in the season before and the season after that run. Neither Manu Ginobili nor Tony Parker has even made the Third Team All-NBA.
George Gervin was in the top 11 in NBA MVP voting in every season from 1976-77 through 1982-83 (he finished 11th in 1977, 2nd in 1978 and 1979, 3rd in 1980, 5th in 1981, 6th in 1982, and 9th in 1983). Tony Parker finished 9th in 2006 (a very good showing) -- but he'd have to improve on that this year and remain in the top 10 for each of the next 5 seasons to have a run like the Iceman had. Manu Ginobili has never received a single vote for the league MVP award.
Mostly, though, George Gervin never got to play with a superstar player (other than himself); Tony Parker and Manu Ginobili have played with 2 no brainer Hall of Fame players. Judging Gervin relatively poorly, compared to Ginobili and Parker, simply because Gervin's teams didn't win les is ridiculous. It's like saying that Robert Horry was a better player than Karl Malone or Charles Barkley. Nobody in his right mind would make that argument.