XSI Mod Tool
There is a discussion of how to use the XSI mod tool here to create models for FSX. XSI Mod is a freeware tool from Softimage, see this page. There is clearly room for someone to take these instructions, deal with the animations issue, and make a plugin to automate this.
Decapod has been making a plug-in for 3D Canvas, which is $70. You can see his work in progress here.
See Abacus' Flight Sim Design Studio (FSDS) which is $50. It supports generating true FSX content. Even though it does not natively support all the advanced FSX rendering features, you can get the freeware FSDSxTweak to enable those in FSDS models. FSDeveloper.com has a wiki page about FSDSxTweak here.
There, 1 free and 2 reasonably priced tool alternatives to GMax.
Very cool stuff, this proves there is and will be life after GMax.
Thanks to Dave Nunez for pointing out the FSDS information