Most medical schools won't care where you got your high school diploma. They will care if you do well in your university classes. Get into a university, do well, and then worry about getting into the medical field. Oh...and once you get into college don't start out taking all the hard classes at once just to show how impressive you are. They don't care...really. You have a higher chance of burning yourself out from all the tough science courses. Pace yourself with everything. You'll be fine if you persevere.
If your applying for graduate school, I don't think most grad schools care so much about your highschool as they do your undergraduate marks+mcat+interviews etc etc. Most,if not ALL universities require you to have graduated with acceptable marks in core courses such as math,chemistry,english and biology. That being said, GEDs are NOT acceptable forms of graduation in Canada. If your applying for undergrad school the best advice anyone can give for better results,is academic upgrading.
I completed up to 11th grade. I will give you I was an honors student. I had transferred schools and to complete my senior year, I would have had to take 3 courses, two of the bs courses I didn'tn want to take anyway. So I enrolled in college, took 12 hours, loved it and then took my GED (this was not commonplace back in the good old days).
The point of the matter is, no one will care about your high school stuff. They will look at your undergrad, and specifically your core medical school requirements.
So, if you are a GED person who wants to go to medical school, do well in college, do well on your MCAT.