How would you tactfully and nicely ask for only gift cards on an invitation for a baby shower?
It is not polite to mention gifts on an invitation. What you can do is send those invitations out early and make sure you put your telephone number and ask people to call with any questions. If people call to ask where she is registered or what she needs, then you can tell them that you are giving a gift card as she has many baby items already. Her close friends will already know this and will know what to do (gift cards, cash, etc.). I went to a wedding about a year ago and friends of the couple just more or less spread it around that they needed no more household items. Only a couple of people brought gifts; everyone else gave cash or gift cards. One of the people who brought a gift was the grandmother of the groom. She told me she got a silver jam server out of her closet and polished it and gave it to them. In the same conversation she showed me a watch she had bought for herself the day before for $400 so money wasn’t the issue.