In "The Metamorphosis," how does Gregor respond when the servant calls out to him?

I wouldn't say that the charwoman is a servant, exactly: she's hired to come in and do some household work when Gregor's family can no longer afford their full-time maid. (The "char" in "charwoman" means "to do random little jobs, usually within someone's household.")

Anyway, about halfway through the story, the charwoman makes a habit of calling out to Gregor in an insulting way, calling him an "old dung-beetle." In response, Gregor slowly gets more and more annoyed, trying to ignore her at first--but then his irritation and anger provoke him into walking toward the charwoman in a threatening manner. However, it's a conflict in which Gregor comes out the loser. The charwoman isn't scared, and she equips herself with a chair against him in an equal show of menace. Gregor backs off.

For reference, here are some excerpts from the story that show exactly how all of this happens:

[The charwoman] wasn't really repelled by Gregor. ... she never failed to open the door slightly every evening and morning and look briefly in on him. At first she would call to him as she did so with words that she probably considered friendly, such as "come on then, you old dung-beetle!", or "look at the old dung-beetle there!" Gregor never responded to being spoken to in that way, but just remained where he was without moving as if the door had never even been opened. ... Gregor was so resentful of it that he started to move toward her, he was slow and infirm, but it was like a kind of attack. Instead of being afraid, the charwoman just lifted up one of the chairs from near the door and stood there with her mouth open, clearly intending not to close her mouth until the chair in her hand had been slammed down into Gregor's back. "Aren't you coming any closer, then?", she asked when Gregor turned round again, and she calmly put the chair back in the corner.

As you can see, the way in which Gregor reacts to these slight taunts reveals how defeated he is becoming. The charwoman works for his family, so she should be below Gregor, socially, and yet his transformation into a low creature himself means that she can get away with treating him as if he is lower than she is. Gregor was once the breadwinner for his family, and now he's completely cowed by the threat of a chair wielded by a household worker.