How can I send gpg encrypted mail automatically from the linux command line?

try something like

gpg -ea -r "Recipient name" -o - filename | mail -s "Subject line" [email protected]

to send an ascii-armored, public-key-encrypted copy of the file "filename" to a person named "Recipient name" (who is in your gpg keyring) at email address [email protected] with the specified subject line.

or

echo "Your secret message" | gpg -ea -r "Recipient name" | mail -s "Subject" [email protected]

to send text directly rather than from a cleartext file on disk.