I don’t know the last year for the Gentleman or Super Gentleman, I’m guessing the early/mid 80’s. The rear dropout width is no more than 126mm and probably 120mm if it is from the 70’s.

To fit any 8 or 9 speed the drop out need to be 130mm wide. One of the primary advantages of a cassette over a freewheel is the location of the drive side bearing. On a cassette it is out board and a freewheel is in board. This means the axle with a freewheel flexes between the bearing and the dropout. The wider the freewheel/dropout the more it flexes and eventually bends.

Depending on how much the bike means to you, you might be at a bit of a crossroads. You might be able to use a triple crankset for more range, or you might want to spread the dropouts and modernize the entire drivetrain.

John