B737-800 Captain reported an aircraft fuel leak while starting taxi on the ramp. The aircraft safely returned to the gate; then maintenance troubleshooting introduced smoke and odor to the cabin; while more fuel continued to leak on the ramp.

Narrative

After normal pushback and both engines started; we had just begun to taxi on the ramp to spot 2 when another aircraft flight crew notified us that fuel was 'pouring' out from under our #1 engine. We stopped at spot 2; communicated to ground control that we wanted to effect a 180 degree turn back onto the ramp and ask for Airport Rescue & Fire Fighting to come and inspect for a possible fuel spill as we had already secured the #1 engine. With fuel still spilling from under #1 engine; company and Airport operations directed us to park the aircraft. After deplaning all passengers; I requested a debriefing with the entire crew in the aft cabin. During this debriefing; maintenance personnel did dry crank the #1 engine. I saw an extremely large amount of white smoke come from the #1 engine. The First Officer and I remarked that a great deal of fuel must have contacted a hot engine section and vaporized. A few moments later we smelled a very strong odor of fuel. I directed that we should get off the aircraft immediately and as we made our way forward; a couple of people said they felt lightheaded. The four flight attendants exited the aircraft; up the jet way and into the terminal. The First Officer and I remained in the jet way for a few more minutes but we both began to feel a headache and so exited the jetway as well.Up until maintenance personnel decided to motor the engine; there was no fume/odor event. Whatever the presumption; a large amount of fuel was introduced into an engine that was hot which caused it to be vaporized and subsequently drawn into the APU intake and thence the aircraft air-conditioning. I've considered what might have occurred had the ignition been on.For myself; I don't think I'll remain on board when maintenance 'troubleshooting ' is going on and I'll communicate that to my crew. We all make mistakes; and I didn't see this coming. I did see a trail of spilled fuel from behind our departure gate to the spots we stopped prior to reaching our parking gate and indeed still dripping. I watched as maintenance was mopping fuel out of the front and rear of the engine as they spread containment pads on the ground. It didn't occur to me that they'd motor an hot engine full of fuel.