I was accidentally given free friends and now I want to ask for a side of ranch.
Could you explain how you got free fries?
I accidentally grabbed the wrong bag.
If they're not your fries, you should tell the restaurant about the error. Most likely they'll realize the accident and will let you keep them, however, I would offer to pay if you can. Or give them back. If you do get to keep them, you should definitely ask for a side of ranch. That's delicious.
I thought it was a gift.
You said it was an accident, so I don't think you should it was a gift, however it's not a huge deal and you should just let them know. Mostly likely it will be shrugged off and you'll get the ranch, too.