I say if you're craving it, have it. It wont go out of your head otherwise! Once shouldn't matter too much. My craving have been all over the health scale, the latest being asparagus and Lucozade (Not together), but last week it was nachos with lots of cheese. Ate it once then the craving went away.
You can balance it out with a healthy breakfast, lunch and snacks. It's sort of like dieting, a treat wont hurt you.
Enjoy your chips and gravy, lets face it, up to now being pregnant hasn't been all roses! xxx