To play Jet Force Gemini on your PC, you first need the game itself. A N64 game on its own is obviously not compatible with a PC; however the data contained within its read-only-memory (ROM) can be read using special software called an emulator. Thus, the emulator needs a copy of this ROM contained within a file it can access. Below is the ROM for the US version of Jet Force Gemini:

For Jet Force Gemini, there are really only two emulators that can properly emulate it; namely, these are Project64 1.6+ and 1964 1.1. Project64 1.7 with Jabo's Direct3D8 1.7.0.53 video plugin does, by far, the best job of properly emulating JFG. Unfortunately, Project64 1.7 is likely never to be finished or released to the public, but you can still donate to download the current beta version. 1964 1.1 is best for the utility side of JFG emulation, such as geometry extraction and code jumping. Links to each emulator are below:

These plugins below will allow for geometry extraction. Unfortunately, neither of them can completely dump Jet Force Gemini. Each has its own flaw to prevent perfect extraction. LemD3D8 dumps inanimate geometry perfectly, but animated things just become a mess of polygons. TR64_OGL extracts everything, but severely distorts the entire scene based on the position of the camera. Extracting the geometry is a very simple process, but it might not be straight forward at first so watch this tutorial.