To me, acceptance is in two pieces, and is based on my coming to terms with my gynecomastia and what I have observed with fellow gynecomastia sufferers.
Personal Acceptance - Whether you developed due to pubescent hormone imbalance or due to medication, there is always a time period, even if only for a very short time, when there is no acceptance at all due to the conditioning we all have had. Men are masculine and women are feminine and and the two shall never cross and now you have a very feminine trait that defines the very essence of femininity, breasts, on your chest. We are told "men don't cry", "men are rough and tough", "men don't wimp out", etc. But there are these two Mammalian Protuberances prominently on display and that doesn't reconcile with our upbringing. There eventually comes a point when you have to at least acknowledge them to decide what the next move is. Reduction or acceptance. For those of us who frequent here, for various legitimate reasons, we have chosen to accept them. Some do not have a choice but to accept them due to the medication or health reasons they have developed in the first place. That acceptance ranges from do nothing to wearing support full time depending on the individuals needs.
Societal Acceptance -Society in general does not look favorably on men who have breasts. This adds stress to an already stressful situation. In my own case, I have come to the conclusion that if universal acceptance of my chest means I would have to submit to the silly taboos women must deal with their breasts, I would be willing to do so. That is better than having to go around hiding my chest as if I am the one in the wrong, when clearly it is society that has the hang up with boobs on guys, not me. Until we can get society as a whole to at least acknowledge having gynecomastia does not make you a freak or monster or whatever, we will continue to be outcasts.
Even women in general have no idea what gynecomastia is. I was called a trolling perv by a woman in another forum because there was no way a guy could have naturally occurring 38DD/36DDD breasts. I then spent the next hour or more explaining it was completely possible and there were men with larger breasts than mine and how it was possible. In the end, she admitted she had no idea this was even possible and finally apologized. I truly believe it is the lack of education is the root of this problem, but that is a very long term solution.
These are just my opinions from observation and what I have personally experienced. Your mileage will definitely vary.