A generator set consists of a prime mover (engine) and an alternator. I think that a 50 kW alternator will be a synchronous alternator. Once one of those is connected to the grid, it will hold the speed and you can put any amount of power up to the rating into the grid.
As I understand it, the power produced by the engine varies. If there is a conventional load connected, the power variation causes speed variation.
If that is the case, once you connect to the grid, the power variation from the engine will cause power variations but the speed will stay constant. That is because the load is effectively infinite.