I'm a little concerned at how interesting I find this and talk of language in general but also @MarriotBonvoy reading my original reply back it sounds really arsey and I didn't mean it to come across that way, it was meant to be jokey so apologies, I wasn't trying to be awful so sorry if it comes across that wayI "think" it;s dialect. Gan has roots in from long ago viking/scandinavian influences in the area. It's not slang definitely. (If I remember what my husbands uncle used to tell me correctly, and indeed if he was right. Assume he was because he did some of his degree on the topic - but it was back in the early 70s) If you go to an scandinavian airport you can see inganned and ootgannen (or similar - totally going from memory which is lazy becauseI have the whole of google)