you can do some computations by using ICGEM official website such as geoid undulations, height anomalies, gravity disturbances, and many calculations by selecting your study area and the characteristics of your study area such as the tide system, the website of ICGEM is http://icgem.gfz-potsdam.de/tom_longtime.
There is also another software provided from the US called AllTrans EGM2008 calculator, you can download it from the website https://www.softpedia.com/get/Science-CAD/AllTrans-EGM2008-Calculator.shtml.
Notice: ICGEM can be used for all GGMs computations, AllTerns for only EGM2008.
MY recommendation for you is to use the ICGEM website for GGMs calculations and don't use AllTrans app because it's very old the ICGEM is covered everything.
here I attached some screenshots that can assist you with what you want.
I hope that is helpful and wish you all the best!!!!