can I change my decree, to not give my ex my pension if I can show proof of dissipation of assets; through her gambling

Probably not, at least directly.  You don't say how long ago your divorce was, but normally property division terms (including pension divisions) are final 6 months after a decree is entered, unless there is some kind of fixable mistake in them. Normally, the time to prove "waste" claims is during the divorce, so it may be too late to make a claim, but if you are not certain, I suggest you consult with family law specialist counsel; sooner than later would be better.