What model year WH?
I guess that the water heater is cold, and not already heated by the electric element? Once the electric element gets it up to 140, then the gas will not run at all.
IS there any flame being produced other than the pilot?
Perhaps the small oriface between the gas valve and the end of the burner tube is plugged. You can use a paperclip to clean it out. Spiders tend to make a web right there. You might have to remove the pipe, or sometimes can wiggle the paperclip into the end of the tube (the fire end) without removing it. Usually it is only held in place with 2 screws, so quick to remove and put back once clean.
Fred.