I'd strongly question why you'd want to do this, but since you've asked the question, yes, I think this should be possible:
- Go through T5 security and into GF, using your BA boarding pass.
- When you've done whatever you wanted to do in GF, take the T5 transit train to satellite C.
- Stay on the train at C, and you'll flow into the arrivals stream.
- Follow the purple signs to Flight Connections, showing your AF boarding pass to whoever is interested (note that you're heading for T4, not T3 as in the OP!)
- Get treated as if you were a genuine connecting passenger transferring onto your AF flight.