Another thing to note about FPSC > GG. The meshes and textures are generally in separate folders altogether (As Wolf has mentioned). It can be quite tedious to connect them all again as once you move all your assets' components into the entity folder, you'll need to update every single FPE file associated. I've done most of this work for people
here.
However, your question is about Prefabs, so you'll need to take the segment tile mesh and copy that over and then generate a new FPE file referencing the FPS file. Also will require moving textures around as well. Again very tedious. Game Guru, however, does have an okay (has its faults) tile grid-snapping mode. Which is great for 100x100 tiles like segments. You will find walls are a complete annoyance however as you may need to run them in Fragmotion to change their anchor points just so that in GG they snap properly.
Door corridors do not convert over. Because GG does not support Boolean hole punching, unlike FPSC. So you will most likely have to create corridors yourself by modifying an existing segment wall mesh. Additionally, the door frames are not part of the corridor meshes nor textures, as the FPS file in FPSC is what groups the frame to the corridor (wall). So you will have to also make the door frame an entity too.
Just a quick test I was doing with some FPSC textures on newly created wall meshes and corridors (+frames):
For what its worth, I'd check out the link Granada posted. I've not personally used it and not sure if it saves off prefabs? But looks nifty.
By the way, if most of the segments you are interested in bringing into GG are fairly flat, you should consider using the EBE (Structure Builder) in Game Guru and you can easily import textures from FPSC to GG to paint on the walls and floors.
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