GMSMA stands for GAY MALE S/M ACTIVISTS [1]. In their own words from a recent newsletter issue, you can get an idea about the purpose [1] of GMSMA from the below:

"Membership in GMSMA should stem first and foremost from the belief that, as a gay male who believes in the legitimacy of sexual expression taking place in the context of an s/m relationship, being part of the group is one way of promoting all that is good and worthy in the organization's pursuit of attaining its goals. Until the creation of GMSMA, there simply had never been a gay male s/m group that attempted in a significant way—on both the national and international scale—the purposes of GMSMA. We were the very first to speak of s/m in terms such as "consensual s/m practices and relationships can be healthy, safe, and positive expressions of human love and sexuality." [1]

It was a member of GMSMA who coined the term SSC around 1984.