Best Answer - Chosen by Voters
You don't specifically mention gift cards. As you correctly suspect, this would be very inconsiderate to your guests and completely against the rules of etiquette.
I would suggest that on the invitation, you say something like,
[Mary] is still stationed in Germany, and will undoubtedly be there when the baby arrives in June. Thus, we plan to send her your best wishes through the mail."
Note that you have not been tacky enough to ask for gifts. People in this day and age know that they should bring something to a baby shower.
By mentioning that you will be MAILING the 'best wishes' [i.e. gifts], only the hopelessly clueless would bring something that is bigger than an envelope. If this happens, exchange the gift for a gift card or cash and send that.