althought I have some Parkings and Gates as HEAVY, I cannot use GSX for B777 & B747, appears the anoying message of parking slot to SMALL.
This has been discussed many times on the forum, the "heavy" designation it's just a label. The scenery developer can use it on any parking, and nothing stops him to make a parking with a radius of 5 mt. and call it "heavy".
GSX only uses the radius, and compares it against your wingspan, so it will consider the parking to be too small if the wings don't fit in it.
Sometimes, there might be reasons why the scenery developer set a smaller size, for example to prevent clashes with jetways from nearby parkings. So there might be enough "physical" space to park the airplane, but the radius in the AFCAD it's smaller then it really is.
In these cases, you can use the "customize parking position" option in GSX, to override the radius in the AFCAD. The little "airplane" icon on the editor page will automatically set the parking radius to the wingspan of the currently selected airplane.