So if they were to stop producing Gmo tomorrow how long would it take for it to stop pollinating in the air and contaminating other farms? I am assuming it will never go away?

You are right, they likely would not go away. The plants that have been contaminated would lead to contamination of other plants and it could go on and on. There is a possibility that it could either eventually be bred out or if this somehow weakens the plants the plants could be more prone to disease that could cause these weaker plants to eventually die out.

I am so obsessed that I found underground societies that are dedicated to non gmo and then I thought to myself well then I would also have to grow my food underground.

There are some groups, such as Controlled Pharming Ventures growing plants in old mines. But growing plants underground would be so difficult. First of all you better have a power plant for all the lighting you will have to install. Then there is the problem of pollination. In nature plants are pollinated by wind and insects, both of which you are going to have  a lack of in an underground farm.

Do you think a greenhouse would be sufficient enough to keep the lingering Gmo's away?

Pretty much as long as you are keeping the pollinating insects out and wind blown pollens.