What would I do? Well, I would never have used it for a DS!
You don't say where the GC is for, so it's hard to answer, but if it's for a multi-department store, I'd buy one toy for him and "buy out" the rest of it for his savings account. If I didn't have the money to buy out the remainder, I'd save the card for things he needed throughout the year (including clothes). My son wouldn't know what Grandma gave him or even understand what that amount is, so there would be no guilt involved.
If the card is a Visa gift card and can be used anywhere, then I'd use it for a membership to the Science Center or Children's Museum or Zoo.
I would never allow my child to have a "field day" at a toy store. To me, that is just wrong on many levels. But we live simply. A foreign concept to most Americans, but it works wonderfully for us.
Blessings.