As a "category" I'm not really that keen on the term Gateway Game.
Yes there are games out there that are great for introducing "the hobby" to non-gamers but at the same time there are, especially now, almost too many games that are seen as "Gateway Games".
That aside I do think there are a number of games which are great for introducing people to the hobby of tabletop games.
I've already mentioned Straw and it's a nice simple game that introduces very light elements of the hobby. It's not alone though.
Fluxx - No matter what version of Fluxx you go with it's a lot of fun. It can suffer from play fatigue if played too often but that's where the other versions come in. The only real criticism though is that it mostly feels like a game of luck rather than a game of strategy. Don't get me wrong it does have some strategic elements but given that the rules and objectives change throughout the game at times it's hard to see any real strategy.
Jungle Speed - A really fun party game which I regularly refer to it as "snap with violence" largely down to the physical element of grabbing the totem. I've played this with gamers and non-gamers and it's been liked universally.
Hey That's My Fish! - Another game that I've mentioned and it has some very simple nods to the strategy and tactical element of area control games. It also benefits from having Penguins in miniature form!
Gloom - At first view given the subject matter you might not think that Gloom is a great intro game. Granted the thought of playing a game about making your family as miserable as possible and then sending them to their untimely death isn't really the sort of "setting" you'd play with kids. It is however is the sort of "setting" you'd play with people who have that darker sense of humour. I've played this with non-gamers a few times and particularly those in older age groups (50+) found it very funny.