Sadly Gary is not with us today. He died back in 2008, aged 69. Gary was a gaming legend. As well as creating Dungeons & Dragons (along with Dave Arneson) then developing the concept into Advanced Dungeons & Dragons, he also founded the company TSR (now owned by Wizard of the Coast) and the Gen Con gaming convention as well as creating/assisting in a number of other game systems such as Chainmail (the forerunner of Dungeons & Dragons) and Lejendary Adventure.
But for me, his most important legacy is Dungeons & Dragons (aka D&D). If you've not played it, its a Role Playing Game (RPG), played by a bunch of friends sat around a table. One person runs the game (the Dungeon Master or DM for short) while the rest take the role of players within the DM's world. The game is moderated by the DM to a framework set by the D&D rule books and rendered in the player's imaginations. D&D is pure social gaming - it is all about the people playing. And D&D is pure multiplayer gaming - it is all about the player's joint interactions. I often feel that all we are doing with digital technology is trying to emulate the purity of gameplay that D&D achieved.
It is also the great-grandfather of much of video gaming. Why? D&D, the product of Gary's rich imagination was based on a key insight to merge character, narrative and stats into a 'game engine'. D&D envisioned a fictional world simulator designed to be played within (and with). This core idea is still the basic framework for so much of today's video games industry. For example D&D has Hit Points and Damage Modifiers, Hit Rolls and Armour Classes - how many other video games, even non-RPGs, have borrowed this combat framework? D&D has progression with character levels, equipment, skills, spells and more. How many games use the various incarnations of levelling up to keep players interested? Tens of thousands. D&D gave roles to the participants via character classes such as Fighter, Magic User, Cleric, Thief and more. How many games use this approach of allowing the player to map their identity into the game? The answer is legion.