This site is pretty good in regards to science and medical articles that are on the same level as the GMAT. Also, the site has a research focus, which is what most GMAT medical passages are on.

https://www.sciencedaily.com/news/health_medicine/

Basically, start reading a few articles from there every day as part of your regular reading. After a while, you should become more comfortable with medical passages. Even better, if the science passages on the GMAT start talking about topics that you've already read about. You'll feel much more comfortable.

Maybe, someone else can suggest other sites that have good medical reading material that's not too difficult.