i designed a gm-cell for a CDR , the i/p was CML standard but i simulated by a large DC sweep signal to find the max. linear range although i was only interested about the standard , the loading was the loop filter , so i think u should use the loading that u have in ur application, other parameters are BW ,linearity, i had a spec on o/p cm so the stability of cmfb and error on the o/p cm, there are some other parameters like PSRR "but it wasnot that important on me", there were other specs but they were mainly related to the CDR application.
regards