My husband was a gambling addict and he stopped. Now he has to spend money everyday he finds something to buy. What is wrong with him?

That seems like compulsive behavior, possibly a substitute for the addiction.

Perhaps just a change of addiction? From gambling to shopping? Missing the "high"?

Some, persons who have broken free from one addiction find something else to do to make themselves feel good.
Sadly in the case of your husband is buying things. What yo7r husband really needs is a hobby in which he can keep himself busy. Something meaning full that he enjoys, Search in the net for hobbies for men. You may find something that may appeal to your husband. Otherwise some people have a few counselling sessions to help them cope with the changes .Communicate with your husband about your worries and try to help him recognise the issue

He's replacing one addiction with another, this is very common with addicts. He needs professional help or this won't stop, but he also has to admit he has a problem.

Your husband has an addictive personality.
Addiction is not just about drugs and alcohol. It's habits that influence choices & these choices have negative impacts on various aspects of their life, health and wellbeing. 
Typical addictions might include smoking, drugs, alcohol, gambling, hoarding, eating disorders, obsessive behaviour .. the list goes on but usually are accompanied by anxiety, depression, impulsive risks, emotionally distant
Support & encourage him to get counselling as this will continue to impact on various areas of his & your life.
Good luck. 

No impulse control.

He obviously has too much money.

Honestly maybe he just needs a new hobby in life. I usually spend my money on buying car parts to fix my old car. And or motorcycle helmets. Give him a new hobby.. one that is not expensive. Lol like maybe going to new restaurants and or making a list of all the places he wants to visit and eat at. Its better than gambling.