GMC is very important in romance novels. It gives the story structure. Goals make the characters seem purposeful, like a character aiming to re - kindle an old love. Motivations add depth, say a character is motivated by past regrets. And conflict creates drama, like when family disapproves of a relationship.
In romance novels, 'GMC' could potentially stand for 'Goal, Motivation, and Conflict'. The goal is what the character wants to achieve, like finding true love or getting married. The motivation is the reason behind that goal, perhaps loneliness or a desire for a family. The conflict is what stands in the way of achieving the goal, such as a rival for the love interest or personal insecurities.
I don't have enough information to say for sure. It could be fiction, or it could have roots in reality that have been adapted or exaggerated for a particular purpose.
The size of the weights lifted in manga can be all over the place. In action and fantasy manga, you might see characters lifting impossibly huge weights. But in more realistic ones, they're more in line with what's physically possible for humans.