They got the gestational age definition incorrect. Gestational age is the same age as the one calculated from your last menstrual period. Gestational age factors in the two weeks before ovulation and conception that your body is preparing for pregnancy. Fetal age is gestational age minus two weeks. My prenatal specialist told me this and if you google gestational age it will tell you that gestational age is the same age calculated from your last menstrual period. Usually ultrasounds measure a few days younger and always use gestational age (but it will indicate on your results what age was used) so if it matches with your LMP but is a few days off they use the date from the ultrasound. They do this because they want avoid induced labor and premature births when your due date comes. They use gestational age because it is difficult to tell when the exact day of ovulation and conception is. Every doctor knows that a woman ovulates roughly half way through her period cycle. The cycle begins on the first day of your last period.
Jennylovell12824 about Pregnancy & Baby App: The Bump, v4.2.10