I didn't know is the Dexcom G4 actually an implant, because it comes out from your skin. Devices like this are pretty common with diabetics nowadays
I was assuming OP was referring to custom-built projects. I too have a CGM (thanks to the recent Australian Federal subsidy) - a massive shame the technology isn't more open.
If using the Dexcom G4, you need to solder one of these to use it with xdrip