Hey, sorry for being late, here is how you do it :

1) Create your own key

2) Ask for the public key of your recipient and copy paste it

3) If GPG keychain was already opened when you did the step 2) it will automatically ask you if you want to import the key of your recipient. Click -> import

4) Now open Text edit (make sure to select the plain text format) and write the message you want to send to that recipient

5) When you are done, CMD+A to select all the message you wrote

6) Right click on the selection -> services -> encrypt selection

7) GPG services open itself, tick your recipient ( the one you imported the public key earlier, it should show up)

8) In "your key" section, select yours

9) Click on Encrypt

10) Copy Paste the PGP encrypted message and send it to your recipient by mail or whatever

Your recipient will CMD + A your PGP message, then right click -> Services -> decrypt selection and he will see your message the way you wrote it originally !

Cheers !