If its a provider you don't know and they are asking for a gift card info upfront BEFORE you meet her, its a scam.
If its a reputable provider and she's asking for you to give her a gift card instead of cash WHEN you meet, its legit. Many advertising sites accept specific gift cards as a form of payment, so if she doesn't want to use her actual cc to pay for ads, it might be convenient for her to have you get it since its something she'll need to buy anyway.