re: If you had one chance to eat lunch in San Diego's Gaslamp district,

old town has the authentic Mexican places but that is about 20 minutes from downtown area

Little Italy has some nice hole in the wall type places, very good food

Gaslamp is touristy

Take a boat cab to Coronado for some good eats as well especially as you get closer to Coronado

I went about 10 years ago and found the seafood to be kind of bland. The Mexican I did eat in the Gaslamp District was very good. I'm not sure if any of this helps being that I don't recall the names of the places being that it was so long ago. That's just what I remember. Gaslamp is a cool area.