As explained many times already on the forum, and of course on the manual, the "Heavy" designation doesn't mean anything. You can designed a parking as Heavy and set it as small to fit a Cessna only. What's counts is ONLY the parking radius set by the developer. GSX compares this against your wingspan, to indicate a parking is too small for you.
And of course, as explained on the GSX manual too ( Page 24, the explanation of the "Max allowed wingspan" parameter of the parking customization page ), you can customize this using the GSX included parking customization feature, that will allow you to override the radius set in the scenery by the scenery developer, if you believe it was set too low for that parking. The small airplane icon on the GSX editor will automatically set a parking to be large enough for the airplane you are flying.