You can use the GPS to swing the compass but you need to account for the local magnetic variation by setting your GPS to magnetic. If it only displays true heading you need to steer for the magnetic heading by adding or subtracting the magnetic variation. If west mag var add to the true heading and subtract if it is east mag var. i.e if you are trying to steer magnetic 0 and your mag var is 10 west you would sail 350 on the GPS. Adjsutments should be made at 90 then 180 then half of the the error at 270 and 0. then swing every thirty degree and record the differnce between your magnetic heading and the heading on the compass--this would be your deviation.