GPG and PAR2 error correction data from the plain archive, will it compromise security?
You could GPG-encrypt both the .7z archive and the PAR2 error-correction data.  That will take care of the security issues.

Alternatively, you could compute the PAR2 error-correction data on the results of the GPG encryption, for the reasons Ricky Demer explains.  That would be secure too.

Either one works.