Hi Ravjeet,
Are you following the below blog for the decryption. You need to decrypt using your private key, i guess you are trying to do it using Public key.
Please follow the below blog to do the encryption and decryption -
Using PGP in Process Integration
And if you still feel the issues is with the keys, then you can try generating the ASCII armored keys from the below link and use this in your configuration.
Generating ASCII Armored PGP Key Pairs - Process Integration - SCN Wiki
And please note, these keys need to be stored in the folders where your SAP application is installed.
Regards,
Nitin Deshpande