Merge new field in email message

Hi, I have created a new field 'gender' with options 'male' and 'female'. During checkout customers have to select an option, all works fine.
But now my customer want to have this field added to the administrator email notification which is send after a new order has been placed.

The name of the field is 'gender', see attached image.
How can I add this new field to the email notification?

Thanks in advance, Burt