I thought once you selected a "heavy gate" it would always work.
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.
When I was pressing Ctrl = F12 nothing was happening as a result.
Yes, of course. I think the verbosity slider should be set AT LEAST to the first notch, so you won't miss warnings like that.