I suspect the gown is as much to protect the doctor as it is to protect the patient's modesty.
The 1st time I had a skin exam with my lady dermatologist, the assistant who brought me into the room told me that because i was male and the doc was a female, that i was to wear a gown. Even though I was also told to leave my underwear on. So that was explicitly for the doctor's sake.
After she had gotten to know me from repeat visits, & the various assistants would just say "Here's a gown", I stopped wearing it & would just sit on it instead. Especially if I am there to have my skin examined, it seems ridiculous during a skin exam to have the skin covered. Eventually i minimized my cover to just a swim supporter jockstrap, so it would have the thinnest waistband & leave my butt cheeks exposed. After all, when i used the tanning bed for x number of years, I would also tan my butt & just cover my genitals.
On a separate note as far as the doc's modesty or whatever, when disposable gowns were first left out for me to wear during annual physicals at work, I had already been seeing this male doc for annual physicals for the past several years. Why was I going to wear a gown now? It seemed silly that it would just get in the way, & I'm as covered and exposed as i am at the beach or in my yard anyway. What was there to be modest about?
Needless to say, i never bothered putting it on, even as the docs changed in the years to come.
Now that I have my own primary care doc & she is female & I am asked to wear a gown, I comply, albeit leaving the gown wide open. I hear her husband who is also a doc there, instruct his patients when they are brought in from the waiting room "shirt off pants on", but no mention of a gown. So that would also lend credence to the "protect the doctor as much..." comment.