Believe it or not, I've been writing seriously for five years now, but it has only been in the last couple weeks that I have learned about GMC. What really surprised me, however, was the number of people in the writing circles I frequent who had no idea either. This included many published writers.
GMC stands for Goals, Motivation, and Conflict, which must be in every story for it to succeed. The concept is actually very simple. The Goals are what the character wants, Motivation is why he or she wants it, and Conflict is what happens to interfere with him or her reaching the goal.
I think the reason many writers do not know what GMC is can be explained by using an analogy of auto mechanics. Many people learned to work on cars by attending formal vocational training and many learned by tinkering with cars in their parents' garages. Some started in their parents' garages and then took some formal classes to expand their talent.
Ideally, every writer has a Bachelor of Arts in English or Journalism and a Master of Fine Arts on top of that. Someone with that education would certainly have learned about GMC very early in his or her studies. Those of us who do not have English degrees have likely not had formal training in GMC. We learned writing on our own, making mistakes and coming back smarter.
Most of us who are not specifically trained in creative writing come up with GMC for our main characters anyway, not being aware of what we are doing. So, if everyone eventually comes up with GMC for their characters, why talk about it at all? The reason is it's easy to get off track if we are not consciously following the GMC for our character. Using another analogy, while the captain of a ship may know the exact route to his destination and sailed the course for years, he still must monitor his course or he will tend to drift off it. Likewise, even the most knowledgeable writer must be aware of the Goals, Motivation, and Conflict of his or her character or he or she runs the risk of the character going off course and becoming less realistic and relatable. This is an area where seasoned and educated writers are in the same peril as newbie and self-taught writers.