Personally I think that's a weird question.
As sypher said, Gmax is 3DS Max 4 stripped down, and without the rendering engine (that costs the money) but then it is freeware.
3DS Max 6 is the lastest out, and is leagues ahead of gmax in the things it can do, only it costs a lot of money if you buy it. Not that I'm suggesting other methods of aquisition

You can certainly import W3D into 3DS Max, most of the gmax importers being written in MaxScript so they also work in 3DS Max.
I'm pretty sure there's a 3DS>W3D exporter out there somewhere, but the download is more elusive than an individual grain of sand on a beach.
