You need to ensure you are sending your message with UTF-8 encoding:
Send with UTF-8 encoding
Each time you send a message, Gmail automatically selects an
appropriate encoding for the language(s) in which you've composed your
mail. It's possible, however, that the recipient may not be able to
properly view the message you've sent.
If your contacts are having trouble viewing messages you've sent them,
we recommend using 'UTF-8' (Unicode) for all outgoing mail. UTF-8 is a
standard encoding that's accepted by many email clients.
Here's how to use UTF-8 encoding:
- Click the gear icon in the upper right, then select Settings.
- Scroll down until you see the
Outgoing message encoding: section
- Select Use Unicode (UTF-8) encoding for outgoing messages
Taken from this Google Help Page