There are no problems whatsoever with GSX and GSX cannot prevent the sim from starting. It will surely slow down the startup so, you might *think* the sim has stopped loading, but it hasn't. Check the manual, Page 97, the paragraph called MSFS takes longer to start with GSX installed, which offers a suggestion to reduce the startup time.
And of course, be sure the whole Addon Manager folder (where you installed GSX) has been added to the antivirus Exclusions, because not doing it, will surely increase further the startup up, making you assuming MSFS is stuck.