GrowFX is a 3ds max plugin exclusively. It is a smaller program inside of 3dsmax that gives 3dsmax instructions on how to build a 3d model of a plant. The .max file you are seeing is the file that is opened by 3dsmax. It contains the .gfx file, as well as the paths for the image (texture) files (.tiff, .tga, jpg., etc).
the.gfx file is a file you can open using growFX when you are inside of 3dsMax.
the other files are just images. they are mapped to the surface of the 3d model within 3dsMax.
As far as getting these files into a lightweight program. I think it might be kind of difficult to do. Usually the models produced in growFX are too large for a lightweight viewer. I think there are a few people around here who are using it to make smaller models for use in realtime rendering, but the ones in the library are generally too large for that purpose. You would also need to know a bit about 3ds max in order to export the model to a format usable by a 3dviewer.
What would you like your final product to be? That will help me give you some more direction as far as what resources to point you towards.
Hope it helps,