I want to change Gate by GSX. But I can not.Like this.
This is not a problem, of course, but just that you are using an airplane too large for that parking, since GSX checks your plane wingspan against the scenery radius, and will give that warning if half wingspan exceed the radius.
So, you should use a more appropriate airplane on that parking.
If you think the airplane is correct, you can override the "Max allowed wingspan", by setting it in the parking customization dialog, either inserting the radius manually, or by pressing the small "airplane" icon.
All of this, of course, is explained in the GSX manual, on the airport customization chapter, page 31.