To answer your questions, suzannelgnz -
1) Do we find out more about where Tony got the gun from? - All I can say is that we have not heard the last of Doug, the guy who stole the gun and sold it to Tony in exchange for a thousand dollars. This will become a topic of discussion between Jack, Richard and Charlie in a later chapter in this fic.
2) Do we focus more on the Holden children's past? - I am pleased to say that later in this fic is a chapter with the title known as "The Story." That chapter is entirely based on the Holden's children's past, but mostly focusing on Tony, rather than Gina. "The Story" was actually my favourite chapter to write, and I'm sure you and the other readers will enjoy it, once we get up to that part.
I don't want to give away too much, but all I can say is, like I've said before, there is still plenty more to come, and every character in this fic will somehow be involved either in the main storyline, or other little storylines that I mentioned in the first post. Hope I answered your question.