cmos gate leakage
3 ways:
1) Some models have parameters for simulations to account for gate leakage. Run a simulation with 0 Vds and a gate voltage and see if the current will give you a value from simulation.
2) Look in the documentation. A lot of design guides will mention their parameters for gate leakage and give you a really simple (empirically determined) formula.
3) ... of course my brain fails me now. There is a simple formula which I believe is the area of the gate * conductivity of the material (something like that... I'm tired). If you don't get better help, I'll dig up the formula for it.