This function performs an if/then/else expression evaluation. It is available starting with GrADS version 2.1.1.b0.
if (logical_expr, then_expr, else_expr)
where:
logical_expr   - any valid  logical expression that has a boolean (yes/no) result
    then_expr      - the result expression if logical_expr is true
    else_expr      -  the result expression if logical_expr is false
All the arguments must be expressions for gridded data -- the logical operators and the if() function have not yet been implemented for station data. 
logical_expr should include one or more of the logical operators: =, !=, >, >=, <, <=, |, &if() function will evaluate logical_expr and wherever the result is  >0 it will place the value of then_expr, and wherever the result is <0 it will place the value of else_expr.then_expr and else_expr may be any GrADS expression, including a constant. If you want then_expr or else_expr to be undefined, then use  maskout() instead of the if() function.
Here is a script sample to find the minimum and maximum 2-meter temperature at each grid point over a 12-month period:
  'define tmin = const(t2m,1e9)'
  'define tmax = const(t2m,-1e9)'
  t = 1
  while (t <= 12) 
  'set t 't
  'define tmin = if(t2m<tmin,t2m,tmin)'
  'define tmax = if(t2m>tmax,t2m,tmax)'
  t = t + 1
endwhile