Anger and the addiction to gamble go hand in hand. An active CG cannot walk away and therefore will always experience loss, leading inevitably toward a feeling of failure, which in turn generates anger. It is possible of course that her mother is angry that you are going through with the divorce but whatever is causing it, there is a lot of anger in the situation and it seems your child is being used as a pawn.
It is good that you are getting support for you, I am sure that is what is helping you to deal more calmly with the chaos around you and long may it continue.
I could not begin to speculate on what is causing your wife to say she wants to give up her child but, while it is so, I am glad that you are making a place of safety for your daughter because she is the saddest outcome of all the problems in your lives.
Breakdowns in relationships where there are children are so difficult James and with your wife’s addiction in the mix it is even harder. Trying to work out whether or not you are being manipulated, or where she is in her addiction, will not help – focus on what is important – and that is you and your child.
You are doing well