i would call gamblers anon helpline and ask for advice on how to explain to your dd what he did and what steps he is taking to help himself - that you split because he did a bad thing -
you can be positive and say now he is doing something to help not do that anymore, he still loves you etc (if that is true)
www.gamblersanonymous.org.uk/family/
i think she is old enough to be told and she needs to know there is a good reason why you not together. my 8 (now 9) is very aware why we cant live together (abusive, agressive behaviour etc)
i dont think you telling him to spend quality time will help - he needs to know that himself. or hear it from someone else probably. eg from her - have her write a letter to him - i would like to send time walking in park with you - or whatever... have her write a list of suggested activites to do with him?