I recently removed the generator and installed a 100A alternator now the GEN light stays lit when the car is running. It was not on before the swap. Is this due to the increased power of the new alternator?

 

Typically the Gen lamp goes on when the current flowing out of the battery exceeds the current flowing out from the generator. May want recheck your installation making sure the same type generator was installed.

 

You said 'Generator', is the vehicle '61 or older?
If you swapped in a alt in place of a gen, you will have to rewire the charge circuit for the alternator.
If the 'gen' is an alt, did it work before swapping out?
Picture of new alternator?