If they're kids, Lego Star Wars is a good starting point, assuming they like either Lego or Star Wars.
Apart from that, it's a hard question.
I generally think that adults who'd want to play games already are - they've tried them but figure they aren't their kind of entertainment.
I know no one here's suggesting it, but I've read the concept elsewhere and I definitely disagree with anyone who'd say that phone games like Candy Crush are a gateway game to PC gaming - totally different people play those types of games. I've played it, but it's totally not for me, and I can't see people who like playing Candy Crush having a go at Skyrim and transitioning. There will obviously be some overlap, as with anything, but I don't see it as any kind of stepping stone.
In fact, keep the Candy Crush fans away from PC gaming. If people who are okay with spending money on microtransactions get into PC gaming we'll start getting microtransactions in full-price PC games, screwing with balance to make more mone... oh, crap!
Though perhaps a closer gateway game for adventure games would be Hidden Object games. I see a lot more overlap there.
I can't think of a gateway game to RPGs, maybe something like Mass Effect 2, where you don't have to think of changing loot or exploring and can just enjoy the story while fighting.