I did the Plat Challenge this past Spring on a Y fare from COS-LHR. When I arrived in LHR for the return, I was shown as "Gold" in the system (and boarding pass). When I asked the agent, he told me that I had accured 5000+ Q points on the out-bound portion, so they gave me Gold up to that point. Platinum was accrued on the return. I assumed that had I not flown the return I would have kept Gold status.

I never did register for the Gold challenge, only Platinum.