By gear oil if your mean : Transmission box oil then....
Check for oil level first, if the gear oil is TQ type ( red colour similar to that of brake oil/ power steering oil ) then there is no need to change the oil anyway. If the level is somewhat lower, only then get it topped up by matching it with the oil prescribed in the owner's manual book.
If the gear oil is CL type ( like thick grey diesel ) then check for viscousity levels ( chiknai ) and if it has already turned milky ( like cutting oil ) then immediately change it coz water has found its way into the gearbox.