how to set imported gpg key to primary gpg key?

Give more details about your problem e.g what you are using the gpg key for and what exactly are you aiming to achieve?

I have a server in american and i have a pc in iran. and i have a personal gpg in my pc,
and a backup gpg code in my server in amrica.

i want to coding my backup in server with backup gpg code and open it in my pc with imported backup key
but when do thid action, see this ERROR:

[GNUPG:] ENC_TO 5F3A77EC6B189DC1 16 0
gpg: encrypted with 1024-bit ELG-E key, ID 6B189DC1, created 2005-05-08
"backup <backup@farsilinux.org>"
[GNUPG:] NO_SECKEY 5F3A77EC6B189DC1
[GNUPG:] BEGIN_DECRYPTION
[GNUPG:] DECRYPTION_FAILED
gpg: decryption failed: secret key not available
[GNUPG:] END_DECRYPTION

NOTE: i signed backup gpg key to my personal gpg key.

why you cant look at man for help.
man gpg or gpg -help