Garry's Mod has such variant gameplay that it would be more advantageous to focus on the gameplay within certain constraints- the primary constraint being gamemodes. Even at that point, it would only serve to explain to people what gameplay within a specific gamemode is like, which can be done by simply searching YouTube for the name of that gamemode. Because of the simplicity of doing this without a dedicated forum combined with the lack of an audience that enjoys videos of nothing in particular going on, I am going to vote "No" to this request, as said dedicated forum would merely rot away.