George Gervin is the 3rd greatest Spur of all-time.
He was the franchise player for 11 straight seasons, and in that time, led the Spurs to the playoffs every time but once, made the Conference Finals 3 times (where his teams had the unfortunate luck to run into the Showtime Lakers twice), led the league in scoring 4 times, and led playoff scoring 6 times.
Whether or not Tony Parker is a better player in a vacuum because he plays in the modern age is irrelevant. Parker has never had to carry the load of an entire franchise as its best player, and if he did, it's very, very doubtful he'd lead the Spurs as successfully as Gervin did.
The only fair way to compare players across eras is to compare what they achieved in their own particular era. Gervin was consistently a top 5-10 NBA player in his era, a legitimate superstar player at his peak, finishing 2nd in MVP voting in back-to-back years and top 5 in voting for four straight years. Parker has only flirted with superstar status and never finished higher than 5 in MVP voting. Furthermore, Gervin has several statistically impressive playoff runs. Parker is usually solid in the post-season but often disappears/chokes or has a marginal impact, and has yet have run that you can call impressive.
The Spurs have had only 3 franchise-superstar-MVP caliber players in their history. Players you can build around (relative to the era). And they are: Tim Duncan, David Robinson, and George Gervin.
Parker is in the 2nd tier of Spur greats with Johnny Moore, Mike Mitc , Artis Gilmore, Manu Ginobili, etc.
I'd also take the "DNA challenge" with Gervin vs. Parker, meaning if George Gervin was born in 1982, he'd be a better player than Tony Parker is right now.