Gendry, at first, do not realize what she was doing. Was she trying to serve him, personally? But after a moment, he gathered what this was. Bread... salt. By offering that, it meant she could do him no harm. In Fleabottom, it was much the same, though they could not always afford bread or salt. But once you offered a meal to a guest, it implied a certain level of honesty and trust.

Which is why it was so terrible what the Freys did to the Starts. What they did to Robb Stark. And to the Lady Stark...

Gendry felt a sudden chill and pushed that thought from his mind. He accepted the food and ate a bit, to signal that he was accepting her hospitality. It was a ritual he'd never explicitly taken a part of before. Even now, he was wary. After all, if the Freys could practice such treachery... and if his own uncle could be equally as treacherous with his sorceress...

He chewed and nodded with his satisfaction. "The water was very warm, m'lady. This food is good, too."