technically, you can use any modeling tool you want for making an aircraft... given that you have the appropriate export plugins for exporting to FS

Gmax is generally recommended because FS comes with modules for it, but if you have any tool that can export an .x file such as Panda Exporter, you can convert it to .mdl later and have it in FS

or another idea would be to model your geometry in your program of choice, save it as .obj as then import it in Gmax to rig and export it once again to FS

this way you save yourself the hassle of re-learning to model and just worry about the specifics of FS rigging

bear in mind that some things might be Gmax-specific, such as materials and animation rigging for FSX, so i'd limit this technique to modeling the geometry itself, and animate and do the hierarchy in Gmax