Friend introduces me to others as "This is my friend who had gastric bypass"

So, I've said this before....  

Oh ok, wow. I guess since it's not your friends or family she feels it's ok. The only thing you can do is tell her you are not comfortable with everybody knowing. Just explain that you were hoping that it wouldn't bother you but it does.

Some people overshare pre and post op . In my case low boundaries definitely contributed to my overeating. 

Its a low self esteem issue . She may not want to reveal these things and probably really regrets embarrassing you I'm guessing

If she does it again, actually get snippy with her at that very moment.  You may embarrass her enough in front of her other friends that she may get the picture.

I think lipstick lady's response is perfect. You know she is only excited for you and you naturally don't want to tamp that down. Starting with a compliment will let her know how you value her support. And hopefully make her more open to the critique. 

My favorite advice columnist, Captain Awkward, has given a lot of advice on how to set boundaries with friends, families, and acquaintances.  Here's a link to that tag - https://captainawkward.com/tag/boundaries/ - reading through some of these letters and responses might be helpful to you!

Whatever the reason she is blurting your business, it sounds as if you will have to be direct. "Please do not reveal my surgery or other personal information. I'm sure you understand how disrespectful it is when I have asked you to refrain from sharing."

It sounds as if you have been tactful, and she isn't taking the hint.