I already did a post about this, but I wanted to go into more detail with this theory.
I was reading about Gwaine and his backstory, and I was wondering if there were any reasons in particular that could have resulted in him leaving home. Even if his mother wasn't the best and he didn't get along with his sister it doesn't seem like Gwaine to just leave his family like that. So I was thinking, what if, after his father died and his mother's plea for help was turned away, she married another noble man. If she had found love again and all her attention was on her new husband and she possibly had a child(or two) with this person, it could have resulted in Gwaine running away?. It makes sense to me, and if he didn't get along with his stepfather and felt unwanted it'd be very Gwaine like to just leave it all behind him. I also like to think that he got along with one of his half(or possibly step) siblings and left them to take care of his mother if something happened, so then he wouldn't have as much guilt.
I think that Gwaine's father died when Gwaine was around 13 years old and his mother would have remarried when he was 16. This isn't really relevant but I've always thought that Gwaine in the show when we first meet him is around 25 (or just turned 26), and I think he ran away when he was 21(I'm pretty sure that's the age when he would have been officially considered a man)
(This is a short summary of what I think happened)
Gwaine, his mother, and his sister have been living in poverty for 3 years. Most of the friends they had when they were nobles didn't associate with them that much but one or two did. One of their friends introduced his mother to a noble, they fell in love and after a little while, got married. Gwaine didn't really like, or get along with, his new stepfather (this probably isn't one of the reasons Gwaine hates nobles but I think it could add to his hatred of them unconsciously) and was annoyed that his mother was paying less attention to him and accidentally ignored him sometimes. This noble already had a daughter she was 7 when her father got married to Gwaine's mum.(her mother died in childbirth) She didn't get along with Gwaine and his sister at first but she and Gwaine gradually became friends. Gwaine started to feel more excluded by his mother when she had another son. He often thought of running away but couldn't because he felt like he had a certain loyalty to his family. But, as soon as he turned 21, and was considered an adult, he ran away making his step sister promise to take care of his family.
I also think that Gwaine's step sister could possibly have been a sorceress. In The Eye of the Pheonix Gwaine identifies the Wyverns in a way that sounds like he learnt it somewhere and he seems to be accepting about magic (for example, Euchdag), so it makes sense if he learnt about magic from his sister.
I hope you enjoyed this theory, please comment what you think of it