A transconductance amp amplifies current, so you need to put a large resistor in series with the input for the signal from you generator (since I see no specs for the amp, I can't tell you the best resistor value you should use but start with 100kΩ).
Without a resistor you are likely looking at the open loop gain of the amp, so all you see at the output is some version of a square-wave or oscillation/noise, no matter how small you set the input from you generator.