And Abraham was an hundred years old (cf. Genesis 17:1, Genesis 17:17), when his son Isaac was born unto him. Literally, at the time of bearing to him (ἐν τῷ τεκεῖν) Isaac. Thus Abraham had waited twenty-five years for the fulfillment of the promise—a remarkable instance of faith and patience (Romans 4:20), as Isaac's birth was a signal display of Divine power (Romans 4:17; Hebrews 11:12).
Whether Isaac was born at Gerar or at Beersheba cannot with certitude be inferred.