I've tried the PMDG 777-200 at KIAH Gate E21 ( which cannot handle a 777 because it's too small, but I've edited it in GSX to allow it anyway ), selected refueling and no problems whatsoever, it arrived and it went away after a brief pause, without displaying the default FSX fuel loading page, which is also correct, because the airplane has its own custom loading system.
Perhaps you have a problem with settings, try to reset all your GSX preferences, by removing the following folder:
%APPDATA%\Virtuali\
Then reinstall GSX, using the current full installer. Note that, removing that folder, will result losing any customization to sceneries or airplanes you made, so you might want to *move* it to a different place, rather than removing it. If you haven't customized anything, it won't matter.