It depends on the design environment for which it is being designed:
-In RF/Microwave circles, everything is specified in power. Gain is a power gain, and input/output impedances are specified (i.e. 50 Ohms input, 200 Ohms output, etc...)
-In slow speed analog circles (i.e. Power managment, etc...), things are always specified in terms of voltage gain. The output impedance is low and voltage gain is not affected by the load impedance. This is actually bad for mixers because they are open-loop devices with a high output impedance, which means your output impedance sets the gain (gm*Rout). So what some do is specify voltage gain for a given load impedance, else they specify the transconductance (gm) if the mixer does not have a low impedance output stage.
Greg