The writing in a wedding gift card should acknowledge the couple in some way. The message may be a simple congratulations, or a thank you for the wedding invitation. Wording can be casual or formal, depending on the relationship with the couple.
A simple, casual greeting is "Congratulations to both of you!" while a more formal message is "My sincerest congratulations on this wonderful day." If the bride and groom are from traditional backgrounds, it may be more polite to write "Best wishes" instead. Even if the wedding is extremely formal, the card does not have to contain a formal message; however, a slightly formal, romantic message is never inappropriate.
If the couple is religious, consider a more formal message that references God, such as "May God bless you today and forever." Consider how religious the couple is before including Bible verses and other strong religious markers.
If one member of the couple is a close friend or relative, he may appreciate a funny message. For example, "Thanks for the free food. Best wishes!" Sarcasm does not always translate well on paper, however, so it is best avoided.
If the gift is being sent to a couple in another country, a message such as "I wish I could be there with you" or "I'm with you in spirit on your special day" acknowledges the situation without including an awkward apology.