I didn't want to reply in the comments so I don't spoil it for new readers.
Francesca is the one who organized the shooting.
George exiled her because of what she did by going against his order and attacking Chelsie and trying to get Gabriel. Even though he originally told her to do what she did.
He marked her so she'll never be able to be part of a pack.
The sins of the father (George)
Are the sins of the son (Gabriel)
At least that's what I got from it. We had a discussion about this a while back on the Facebook Roxann Season Addicts page and this seems to be what most of us think. Roxann didn't comment on it though.