Whether you know who he was depends on whether you’re a roleplayer. For everyone else: he’s one of the two people who created roleplaying games. Along with Dave Arneson, he wrote Dungeons and Dragons, back in 1974. He has, therefore, had more influence on my life, indirectly, than almost anyone else; not only was he responsible for the hobby I spent much of my youth on, he was also responsible for the industry in which I built a career.
I didn’t know him personally at all, but this is still a very sad event for me.