Gross annual value of a property which is let-out throughout the year is determined in the following manner :
Step 1: Compute reasonable expected rent of the property (for details refer to FAQ on computation of reasonable expected rent).
Step 2: Compute actual rent of the property (for details refer to FAQ on computation of actual rent).
Step 3: Compute gross annual value (Gross annual value will be higher of amount computed at step 1 or step 2).
Source:- Income tax website.