First of all, GMO seem to be getting a bad reputation, as opposed to the hormones that are given to animals, like dairy cows. In general, GMOs are safe. In fact, modifying plants' properties and features to something that the farmer wants is been going on for thousands of years (it's how we got broccoli from a plant that looked more like kale).
That said, many of the Light Lagers (BJCP Styles 1A, 1B and 1C) have adjuncts like corn and rice (as much as 40% of the total grain bill), both of which are notorious for having many varieties that are GMO. One of the reasons for this is that it allows the brewer to brew a high ABV beer and add water back to bring the ABV down to whatever the local laws say are the maximum without being labeled with the ABV. Large breweries will manufacture in one state and sell their product in many states.
I have noticed that some beer labels list GMO free rather prominently, however, if a GMO free lifestyle is something you're interested in, I would stick to beers that have ONLY Barley, Hops, Water and Yeast as ingredients.
According the Briess, a very large maltster, website:
[B]ecause malt is made from whole grain and minimally processed, it is an all natural ingredient that helps achieve product claims like natural, healthy, Kosher and non-GMO.