If you use GPS-based synchronization you have a reference of when zero crossings should occur. This does not mean that you have to output exactly that waveform.

Once you have a reference you can use a control algorithm. like a PID, to adjust the phase of the grid that you are generating locally.

If you sense that the grid is running out of phase from what your GPS clock says is the true reference you can increase/decrease your power output a bit to increase/decrease its frequency and catch up with the phase error.

It would be analogous to a type 2 PLL. The 1 Hz GPS clock would be the reference clock, the 60 Hz grid would be the "PLL output" and the VCO would be the power turbine, or AC inverter.