Perhaps it is only viewed as a particular sexual fetish, but my own experience with the subject leads me to believe that it is much more than that. Like any form of body modification, there is a great deal of variety in the number of approaches, goals and inspirations for gaining. As a gay male, I can only relate my experiences from a mostly male perspective, although, I would assume, many of the same principals may apply to females as well. Unfortunately, the female perspective is significantly more difficult to detail as there is a much greater bias against such practices. In some cases, the gaining process it very specific.
Many gainers focus their attention on increasing the size of their belly alone.
Such males usually tend to be more dominant personality types with the distended belly becoming a symbol of power and authority. This, I believe, relates to many traditional authority figures such as the father or even such icons as the beer bellied police officer. Some examples I have encountered are no less extreme than any form of piercing or tattooing. There are individuals who have documented their progress and shown such a radical change in appearance as to become virtually unrecognizeable in relation to their initial form. Others take a more general approach with the most extreme cases aiming for levels of obesity which would essentially leave them bed ridden and requiring constant care by others.