HOWTO: Perform file integrity check with gpg
1) Import GNU keys
gpg --import ftp://ftp.gnu.org/gnu/gnu-keyring.gpg // GNU
2) Verify downloaded file
gpg --verify gnupg-1.4.15.tar.bz2.sig // GNU
OR
1)
gpg --verify ./octave-3.8.1.tar.gz.sig
gpg --no-default-keyring --keyring vendors.gpg --keyserver pgp.mit.edu --recv-key 5D36644B
gpg --verify --verbose --keyring vendors.gpg ./octave-3.8.1.tar.gz.sig