Another GFUNC Question

List,

Sorry to keep bothering everyone with these minor questions. I am practicing with creating grids with gddiag and I have ran into a problem. I saw an example for calculating the Showalter Index and I managed to get all the compnents calculated correctly. However...the example I saw said you could use one value if the lcl press was less than 500 mb and another if the lcl pressure is less than 500mb (moist lapse rate versus dry lapse rate). Anyway...how do you tell GFUNC to use one parameter if certain condition is met...but use another if it is not. Do IF/THEN/ELSE statements work or is there another function that needs to be called.

Thanks for any help,

Jeremy Martin
WFO GLD