The gm/Id methodology should be model independent but some models (eg: BISM3) are predicting the gm/iD characteristic not correctly. I would recommend further reading:
P.G.A. Jespers; 'The gm/ID Design Methodology for CMOS Analog Low Power Integrated Circuits'; ISBN: 978-0-387-47100-6
D. Stefanovic and M. Kayal 'Structured Analog CMOS Design', ISBN: 978-1-4020-8572-7
D.M. Binkley; 'Tradeoffs and Optimization in Analog CMOS Design', ISBN: 978-0-470-03136-0
C. Enz, E. Vittoz; 'Charge-Based MOS Transistor Modeling: The EKV Model for Low-Power and RF IC Design'; ISBN:0-470-85541-X