If you're given a gift card, is it OK to use it to buy something for someone else?

IMHO the minute I give you a gift it's yours to do with as you wish.

Not at all, once you give a gift it's out of your hands. Thinking of others besides yourself is an admirable quality.

When you send your thank you for a gift of money, it's supposed to say that you appreciate having the ___ you bought for yourself with the money, not just say appreciate the money. But I suppose if you wanted to say that you used to money to treat yourself to a nice meal, some hosiery, something for your home that nobody would ever challenge you on telling a harmless fib for the sake of being polite. 

i dont see why you couldnt

No, once you give something it is nothing to do with you anymore.

Yeah but it's most likely for fun.

Like any gift received, the recipient of the gift card has the prerogative in how they choose to use the gift card. Additionally, there is no obligation to tell the person who gave the gift card how it is being used.

A gift is something given to you. Now that it is yours it is your choice. If you had burned it in front of them they may take offense, I would too but shouldn’t cause it shouldn’t have expectations on it... but every gift is going to have an expectation on it...so it depends on the person. I would personally be disappointed you didn’t get yourself something cool and also at the same time be happy you did

 A gift card is a gift for you to use however you want.  Usually they're for a specific store or business.

Many people consider gift cards as a handy way to give money to somebody.  That way they don't have to shop for an actual present, wrap it, and include a "Happy ..." card.

I don't think the donor needs to know what you did with the gift card.  If they ask, you can tell them if you want to, or you can come up with a "little white lie".

Actually, I'd think that extremely generous of you to want to help someone else. Go for it!