Almost took me a day to decipher your question as well. I believe you are asking: "Does WeChall send e-mails to me using my public GPG key or using WeChall's own private GPG key?"
If you are, you don't seem to understand how asymmetric cryptography works. Perhaps read up a bit on that.
As for the challenge itself: read the challenge description. It tells you exactly what to do.
I seem to be stuck on this one at the moment as well. Perhaps one of the pros can give me a hint about what in my key export block is incompatible with what the challenge is expecting as input.