B737 pilot reported during taxi they lost AC power after shutting down the APU. They lost both #1 and #2 engine driven generators; restarted the APU and restored AC power and returned to the gate.

Narrative

25 minutes prior to departure a ramper disconnected the GPU without flight deck coordination. This resulted in us temporarily on battery power. We started the APU. No anomalies were noted. We pushed back and single engine taxied to Taxiway 1 north holding short of Taxiway 2 and performed a delayed engine start. We received taxi instructions as my FO completed his flow. Both of us confirmed we had bussed up to Gen 1 and 2 before he shut down the APU. We began our taxi to XXL at TT with no abnormalities. About 10 seconds after my FO shutdown the APU we experienced a total loss of AC power. I stopped the airplane and set the parking brake to figure out what just happened. While my FO coordinated our unexpected stop with ground I ran the (NG) Loss of Both Engine Driven Generators checklist. I matched the lights to the condition statement and actioned the QRH. Neither generator successfully bussed up so we started the APU and successfully restored AC power. We parked in the Pad to coordinate a gate return with Maintenance Control and operations and returned to the Gate shortly thereafter. Dispatch and Chief Pilot were debriefed after shutdown checklist completion.