You gotta be more specific.
Do you wanna use the MosFET(s) as:
... a voltage-controlled resistor?
... a variable-gain amplifier?
... as a pre- or post-amplifier block, with some other device handling the actual gain-reduction duties?
MosFETs have two characteristics which make them tricky to apply in, say, a voltage-controlled resistor configuration: the intrinsic drain-source diode, and the fairly high gate capacitance, which limits turn on/turn off speed. For control of AC signals, MosFETs are usually used back-to-back in a "bilateral" arrangement to defeat the rectification effects of the drain-source diodes.
A MosFET would not be my first choice for a gain control element.