Will I gain weight?

You'd have to eat a pound above your maintenance calories, approximately, to gain a pound of fat. Eating 3500 total, or 3500 above deficit goal, the theoretical gain is less. IME, in practice, the rare well-over-goal eating doesn't usually result in the theoretical amount of fat gain, anyway, in the long run.

In the short run, it's reasonable to expect a scale weight jump of a pound or few that will persist for a day or several, maybe up to a couple of weeks, even a month if it hits your menstrual weight fluctuations just wrong, but it's not all fat, and it will drop eventually.

You ate the food. I hope you enjoyed the food? (I would've! 😉) Don't stress over it, it's history. A rare day over goal is a drop in the ocean. The majority of our days determines the majority of our outcome.

Just go back to your healthy routine: It'll be fine.

Theoretically, you ate enough to gain about 0.5lbs of fat. Even if you're following the slowest rate of loss on MFP, that's only delaying your weight loss journey by a week.
That's really nothing in the grand scheme of things.

PS: guilt is entirely optional for losing weight 😉 I'm not a fan of it and I never feel guilty. It's just food: I eat it, enjoy it and, if necessary, learn from my mistakes and then move on.