Switching MOSFET with isolated gate
The MOSFET turns on when a voltage is applied between its Gate and Source. In your circuit there is no path from the negative side of V1 to the MOSFET Source, so it won't turn on. All that will happen is the entire circuit (M1/Rload/V2) going up and down in time with V1.

You don't need an actual ground, but you do need to get M1's Gate and Source to the same voltage as is across V1. If for some reason that cannot be done by connecting the two 'grounds' directly together  (eg. one side is connected to AC mains, or the MOSFET is switching the 'high' side of the power supply) then you have a few options:-


A level shifting MOSFET driver such as the LTC7001, which creates a local output 'ground' using a charge pump to raise the voltage. This still requires a connection between the two circuits somewhere, but voltage on the MOSFET can 'float' relative to their shared connection.  
An optocoupler which uses its isolated transistor as part of a driver circuit powered from the FET circuit. 
A photovoltaic MOSFET driver such as the Panasonic APV1121SZ, which produces the required Gate voltage directly without needing any circuit powered from the FET side.