Can you test if you just copy the GaN HEMT block from the model I sent into your own circuit (and not pull it from the PLECS library browser)? I see that you are still using the library file (the mask parameters window indicates this with “(link)” in the subtitle), but I wouldn’t know if that library file is exactly the one I posted, or an earlier one (that might have some modifications or corruption).
As a quick test, you can create a blank Test123.slx file in another directory location, alongside a folder Test123_plecs/ in which you place the XML file (this is a trick to directly link an XML file to a specific model file). Then, place a PLECS Circuit block into the blank Simulink model, and double-click to open it. Then go to the PLECS File menu and choose “Import from Standalone…” and choose the “GaNSystemsHEMTComponent.plecs” file I attached earlier. Then, navigate to the XML file, which will be the only option showing since you are not here linking to the library directory. What happens then?