Or HaChaim on Genesis 20:3/אור החיים על בראשית כ׳:ג׳

1 הנך מת על האשה אשר לקחת. "Here you are going to die on account of the woman you have taken." The meaning is "after I reveal to you that she is married you will be guilty of death." Had G'd not informed Avimelech of Sarah's true status as a married woman, he would not have been guilty of death even if he had slept with her. Any Gentile who sleeps with a woman whom he has reason to believe to be single is innocent of adultery. Maimonides explains this in Hilchot Melachim 9,5.

2The verse may also be understood as telling Avimelech that he was guilty of death having taken Sarah while aware that she was married. However, it is more likely that at the time Avimelech had not known this.