How to correct PSMSL data for GIA?
The simple answer is that you just subtract the nearest value from the glacial isostatic adjustment (GIA) model. The values provided by Peltier in your netCDF file are in a grid that is 1 degree x 1 degree. So with the lat/lon position from the tidal gauge sea level data, you choose the closest value to your station and apply the correction. In the PSMSL GIA correction page they already explain that you should be using the centered value (±250 years). If you decide to use the ASCII files, those are already interpolated to the tide gauge station locations. You have additional information in the U. of Colorado GIA websites(1, 2) and in the Peltier (2001) reference.