The Egyptian here used a common term for referring to God or the gods. He wanted David to swear to the God that David believed in before he took him to the camp. Apart from David’s word, he had no guarantee that David would not simply hand him over to his old master in exchange for some kind of reward. David must have agreed to take an oath since the man proceeded to lead them to the Amalekite camp.1
15 And David said to him, “Will you take me down to this band?” And he said, “Swear to me by God that you will not kill me or deliver me into the hands of my master, and I will take you down to this band.”