In my case I 1st and foremost had to ensure that the fuel tank position and height of the PMDG 737 is enabling the GSX refuelling animation to start.
Load a flight with the PMDG737. Open GSX via the ingame top MSFS bar. Open the "customize airplane" menu in the GSX menu, click on "fuel" and the original setting before the change to be made further down below most likely looks like
in the attachment "fuel_hose_snap1.jpg" and the GSX fuel truck refuelling animation in my case simply did not start.
Because the 737 wings are with these position settings not "high" enough for GSX' animated fuel trucks, I needed to "trick" GSX and move the fuel point further out to the wing, so it's higher. This way all animated fuel trucks (with the people moving the hoses etc.) work, also via the ground/hydrant refuelling.
Simply open the "customize airplane" menu in the GSX menu, click on "fuel" and enter the following measures: 14.36 -6.56 3.57. See "fuel_hose_snap2.jpg". Click on "save". Leave the MSFS flight back to main menu. Close cuoatl.exe and restart it again. Start a new PMDG737 flight and check if animation of refuelling process now works. In my case it did and always does since that change.