I’ve never heard of this, but it would probably be an appreciated gesture to the musicians, especially if accompanied with a heartfelt note of thanks. Regardless, it is probably going to feel slightly awkward to receive it, since they are not contracted to perform for you.
With respect to accepting money: I have played in good friends’ weddings, and it is always awkward to discuss money, because I am usually invited to a rehearsal dinner as well as the reception, and often I want to leave them with a small gift and card, yet I do consider my service part of that gift. Often I would receive a note of thanks at the rehearsal dinner with money, and sometimes that would be in the form of a gift card.
Honestly, I always appreciated cash, and/or check, because the giver has placed no strings on how to spend (or save) the value, and I don’t have to spend money to use the full value. Even “VISA debit gift cards” or the like cost money for the giver to activate, and often they cannot be used for certain online purchases, so they cannot be used “anywhere”. In the case of any gift card, you usually have to spend more than the face value of the card to use it up, so often you are forced to spend money to save some.
If someone wanted to offer me $20, and they know I like books and doing music, I would appreciate the gift just as much if it were a crisp $20 bill accompanied with a note about how they know I like to read books and play music, and they hope I can use this toward my goals. (Rather than “Here’s a card for Barnes and Noble or Sam Ash”.) They may not be aware that I like to buy unique used books as I find them, or I often need to order or get service from specialty independent music shops. They may really not be aware that I just need that gift to put that toward a new mattress!