Date of conception + 2 weeks is the gestational age of baby.
Many women have a 4 week cycle and ovulate 2 weeks into it so gestational age is based on LMP but many other women ovulate outside this time frame in which case adding 2 weeks to conception date gives gestational age.
Just noticed your from uk like me,our scans are different from usa etc.your actual dateing scan wont be until your 12 weeks.that way when you get your scan paper results it will show you exactly what they mean.
Your dates are correct what they are saying is your baby is 6 weeks and 4 days.your ovulation day can vary.remember when you ovulate it takes time for egg to implant so it wont actually be day you ovulated.could be few days later egg implanted.so pregnancy wise your probabley 8 weeks + but baby measures at 6wk 4 days.this would be right time to hear a heartbeat.so dont worry.going by your last period this is right.
Gestational age is the time measured from the first day of the woman's last menstrual cycle to the current date. It is measured in weeks. A normal pregnancy can range from 38 to 42 weeks.
http://www.righthealth.com/Health/What%20Is%20Gestational%20Age-s?lid=goog-ads-sb-8536643334
hope this helps