Braindump of some common command.

Import a public key

gpg --import path-to-public-key To change the trust level given to that key: gpg --edit-key <key-id> then trust.

To verify everything is fine: gpg --list-keys

Decrypt a message using your private key

gpg --decrypt --output output-file encrypted-file.asc

Encrypt a message using the public key of the recipient

gpg --armor --encrypt --recipient <key-id> --output encrypted.asc plaintext-source

--armor will generate an ASCII file instead of a binary file.

Sign a message

In gpg parlance, to sign a message means encrypt it with the private key. Anyone with the public key can decrypt it.

gpg --armor --sign --local-user <key-id> plaintext-source

If you only want to prove integrity without encrypting the cleartext: gpg --armor --clearsign --local-user <key-id> plaintext-source

Symmetrically encrypting something

gpg --symmetric --output ciphertext-symmetric plaintext and then gpg --decrypt --output decrypted ciphertext-symmetric

More

See my full gpg setup for setting up the whole thing.