Who has Gazpacho on their menu in Center City?
Khyber Pass Pub has a Green Tomato Gazpacho on special right now. I've never had it there but I have had gazpacho at their sister restaurant, Royal Tavern (same exec chef) and it's awesome there.
Lots of places will have it on special so I don't think you'll have too much trouble.
Two Garces restaurants have it listed on their menu
The rendition at Amada is a white asparagus gazpacho, the version at Tinto is a classic tomato based gazpacho.
Both are Tapas restaurants, Tinto is oriented to flavors of the Basque country, Amada is traditional tapas.
Distrito, a mexican small plate restaurant has a watermelon based gazpacho.
I am sure there are others that are specializing seaonal dishes.