Do ultrasounds measure fetal or gestational age? And how accurate are they at 7 weeks 5 days?

Yes. "Accurate". Ultrasound pregnancy dating scans measure fetal size or age (time since conception), and convert the measurements to human gestational age. Gestational age is calculated by adding 2 weeks to the age of the fetus (to the time since conception), and roughly represents the total weeks since the first day of the last menstrual period (LMP). All dates are ESTIMATES, so 7w 5d means about 7w to 8w 3d.