FG called mistress in the books

"When more than one slave girl stands in a relationship of slave girls, as when they serve in the same shop or house, or adorn the same rich man's pleasure gardens, it is common for the master, or masters, to appoint a "first girl." Her authority is then to the other girls as if that of the Master. This tends to reduce squabbling. The first girl is usually, though not always, the favorite of the Master. There is much competition to be first girl. First girls can be cruel and petty but commonly, they attempt to govern with intelligence and justice. They know that another girl, at the master's whim, may become first girl, and that they themselves may then be under her almost absolute power. In my house I often rotated the position of first girl among my slaves who were native Goreans. I never made and Earth-girl slave first girl. This is fitting. Let them be always as slaves of slaves."
~Explorers of Gor, page 78~



"For example, in a group of female slaves, for example, in a pleasure gardens, a fortress or a tavern, there will usually be a girl appointed First Girl. Indeed, if there is a large number of slaves, there are sometimes hierarchies of “first girls”, lower-level first girls reporting to higher-level first girls, and so on. The lower-level slaves will commonly address their first girl as “Mistress”.
~Magicians of Gor, page 123~



“A switch was put in the hands of Eta. She stood over me. I put down my head. She did not strike me. I looked up at her. I realized then that she was first girl in the camp, and that I must obey her, that she had been empowered to set me tasks and duties. Suddenly I feared her. Before I had looked down upon her. Now I trembled. It was she who held the switch over me. Before I had generally obeyed her only when men were present. I had preferred to leave her the work. Now I realized I must, without question, take slave instructions from her and discharge swiftly and well whatever menial duties she might place upon me. . . . I had little doubt she would use the switch, and richly, upon me, did I not work well. . . . She could command me. She held the switch. I would obey. She was first girl.”
~Slave Girl of Gor, pages 66 & 67~



“It seemed strange to me, that she had addressed me as Mistress, and her fear. Then I realized the legitimacy of her fear, that of a slave girl. She was the one who had called me “Dina,” and who, when I had been bound, had kicked me. Now she was owned by my master, and she was a newer girl than I. She did not yet know the nature of the relationships in which she was now helplessly implicated, relationships which could be every bit as perilous and significant as the physical bond of steel on her wrist. Was I first girl? Was I over her? Did I have switch rights upon her body, as Eta had upon mine? Would I be cruel to her? Would I make her suffer? Would she have to please the masters incredibly, and constantly attend them, that they might perhaps be moved to shield her to some tiny extent from my vengeance?”
~Slave Girl of Gor, page 131



“Am I ready to go out on the floor now?” I asked. The “first girl” in a tavern often inspects her inferiors, before she permits them on the floor.”
~Dancer of Gor, page 380~



“Yes Mistress,” I said to her. I must address all the female slaves in the house of Hendow as “Mistress.” That order would be in effect until it was explicitly rescinded, probably, depending on my behavior and progress, in a few weeks. This is sometimes done with new girls. It helps to keep discipline among us. I would then, when the order was rescinded, be able to call the girls, with the exception of the “first girl,” by their own names. I would be one of them. Tupita was “first girl.” We must all call her “Mistress.”
~Dancer of Gor, page 170~



“I knew that Tupita was having her sport with me, but, too, I knew that she might beat me tomorrow, in the slave area. As first girl she had that privilege. I did not want her to whip me, or switch me, or have the other girls put my ankles over the low bar and tie them there, and then have her spank the soles of my bare feet with the springy, flat board. It is very painful, and it is hard to walk after it.”
~Dancer of Gor, page 211~


“Slave girls compete for the attentions of masters. Each strives to be more pleasing to them than the other. The quality of a slave girl's life is commonly a direct function of her pleasingness to her master. Whether she is a treasured love slave or an ignored pot-and-floor wench depends much upon her. Gorean men, unlike the men of Earth, do not bother much with girls that are not pleasing to them.”
~Slave Girl of Gor, pages 131 & 132~