psht, I posted this video like 2 weeks ago and I think all of 2 people saw/commented on it
You dont 'make' any levels, garys mod has a bunch of preset levels and that's the first one in the list and then after that he just loaded and set em all up....
However if you notice during one part the camera angle suddenly snaps to somewhere different and he's got a machine gun instead of the gravity gun Me thinks his dominoes failed at one point so he continued it from there or something since he couldn't get it to work in one swoop
EDIT: Oh yeah regarding that honda commercial, I don't believe it's shown in the US, but I have seen it because gary that made gary's mod held a contest for people to create a situation like that......however dominoes from what that advertisement was is two different things.....theres a specific name for what its called, but basically everything falls into line and causes something else to happen.......a long process is created in which an otherwise simplistic ask is made into a long and tough one.
People would have barrels explode, that'd cause a merry-go-round to spin, which would pull a rope, which would do this and that and this until it wound up like killing zombies....the longest most creative 'thing' one in that particular contest, but dominoes wouldn't have been considered the same thing.