GnuPG is a complete and free replacement for PGP. You will use gpg command to validate a GPG certificate. For example, purpose download Apahce-web server tar ball. Visit Apache web site to download latest version of Apache. As usual, use wget command:

$ wget http://www.apache.org/dist/httpd/httpd-2.0.55.tar.gz

Next download gpg key listed next to download link:
$ wget http://www.apache.org/dist/httpd/httpd-2.0.55.tar.gz.asc

How do I verify integrity of the tar ball?

Use gpg command as follows:
$ gpg httpd-2.0.55.tar.gz.asc
Output:

gpg: Signature made Monday 10 October 2005 07:05:15 AM IST using RSA key ID 10FDE075
gpg: Can't check signature: public key not found

You will see an error message ‘Can’t check signature: public key not found’. It means you need to get the key (called 10FDE075) and install it in your public keyring. You can download key from free public key server such as pgpkeys.mit.edu

$ gpg --keyserver pgpkeys.mit.edu --recv-key 10FDE075
Output:

gpg: requesting key 10FDE075 from hkp server pgpkeys.mit.edu
gpg: key 10FDE075: duplicated user ID detected - merged
gpg: key 10FDE075: public key "wrowe@covalent.net" imported
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)

Once the key is installed, use the following command to check the certificate of a source code file/tar ball:
$ gpg --fingerprint 10FDE075
$ gpg --verify httpd-2.0.55.tar.gz.asc httpd-2.0.55.tar.gz

Online References:

• The GNU Privacy Guard
• OpenPGP
• How to create your own GnuPG private/public key pair
• Read more of the Howto: Verify integrity of the tar balls or source code