In France in the 50's , 60's and even years after I am quite sure that boys ( I can't talk for girls because I have no sisters ) had never a gown when they were examined as children, pre-teen or teenagers or young men, either at a doctor's office (with a nurse or not present no matter) male or female doctor no matter, and either at school physicals examinations in group.
At school, we were allways instructed to strip to briefs only ( no socks nor watches and so on ) or even at high schol sometimes in the nude after removing our brief to enter in the examination room where thre was a female doctor and a female nurse. So I was stark naked to climb on the scale for the weight, and eyes test and all examinations since the beginning, even questions ; it seemed normal, the Pe teacher who supervised us in the locker room gave to us the instruction to strip completely. At middle school, it was in briefs. at a doctor'office, it was also in briefs generally but no gowns even if the mother was present.
So of course even the idea of a gown for boys seemed stupid. They might be naked immediately or when necessary during the examinations and all adults agreed with that situation.
Even as an adult, I was less often undressed at a doctor's office (except for mandatory job physical examinatioins - like at school, it was at least undressed in briefs, and sometimes stark naked depending on doctors and on locations - if there was a cubicle to undress or not)) but I was never offered a gown.
It was only at hospital that we had a gown.
I don't think that it has much changed for the gown today.
Thre are less and less physical examinations at school and the students are certainly not told to undressin groups and never stark naked not even in briefs sometimes ( to be chest bare can be enough) but I am quite sure that even today they don't have a gown.