I think it should be clarified the difference between elective and cosmetic surgery.
Elective surgery is a surgery not performed in an emergency, but instead scheduled ahead of time.
Cosmetic surgery is surgery performed with a lack of medical necessity, but instead for aesthetics alone.
From your question, I'm assuming you mean cosmetic instead, because GCS is actually an elective surgery. Surgery can be medically necessary but still be elective.
As for medical necessity making it not cosmetic-
Top surgery versus bottom surgery I find is (for me) the easiest to justify to allies with physical medical reasoning (as opposed to mental health arguments, which sometimes people don't think is a real reason). Usually once people accept one, they tend to accept both in my experience.
Long term binding (which is necessary for daily comfort and acceptance as male) is detrimental to rib and lung health. Pre-existing conditions make binding every day for the average work day even more difficult or in some cases life-threatening, such as extreme asthma. Broken, cracked, bruised, or warped ribs are not uncommon in people regardless of health. It causes back problems and chronic pain or physical discomfort. "Not binding" is simply not an option for many people, because that would mean not being able to live as their gender at all.
In addition, as a preventative measure, both top surgery and hysterectomies help reduce the chances of trans people having undiscovered problems in those areas. It is NOT because trans people are more likely to get problems, but the fact that it is so uncomfortable for us to even get those parts examined, and they can easily go much longer unnoticed and progress further before treatment. On top of that doctors have been known to mistreat or refuse to treat trans people when they do seek help with those gendered body parts, and if there's difficulty even getting these areas examined, it can be healthier for many people to simply have them removed and be no longer a concern. There's a lot of things people have removed simply because it isn't a necessary body part and it's more trouble than it's worth without it being life threatening.