well basic things to keep in mind when making a model for FS:
-mesh should not have overlapping faces.
-all faces should have normals sticking outwards.
-meshes must be completely closed (when possible, if not then care to leave no seams open).
From this
source it seems that GSUpro is able to export to collada (.dae) format, this is quite good for you because with that format you can load the model directly from .dae format into .pof (which is FS2's format).
I'm not an expert in this, maybe someone else can help you out, but you'll need PCS2 to convert from dae to pof, I followed this wiki entry to set up my PCS2:
http://www.hard-light.net/wiki/index.php/Blender_to_POF_ConversionsIt's a blender to pof tut, the first entry is a mini tut on how to set up a PCS2 installment.
The others are just general and particular pointers on how to set up a mesh correctly when exporting to dae and then to pof, it might help you reading it.
Ask away anything you don't get, I would recommend trying to export something simple, like a cube first just to test how it works, then proceed to use more complex models... at least that's how I do it.