any ideas to solve this
Creating an airplane configuration for this airplane using the airplane configuration utility included with GSX, of course.
It the airplane is not supported by GSX internally, you will always to this to add a new airplane. It looks like this airplane has both cargo doors in its aircraft.cfg in the same position. You can fix this with the GSX airplane editor. The 757 Freemium is supposed to be supported but maybe there's a new variation which is not recognized. In any case, the GSX airplane configuration editor should work.