Yes, there are many concerns regarding the impact of GMOs on the environment. First, let us understand what a GMO is. This is when someone alters the genetic makeup of an organism by inserting genes into its genome. Thus, compared to more traditional methods of plant breeding, which are more gradual, and limited to species that can breed together, GMOs are much faster and have greater variety. Thus, the difference between a GMO versus a domesticated organism that has been bred is only that a GMO happens suddenly with a wider range of possibilities. I am clarifying this because even more traditionally bred species can have some of the same problems, but are generally lower risk. For example, killer bees are the result of breeding gone awry.
That said, the impact of any GMO is not known until it is released into the environment. Might it become invasive and cause extinctions? Might it be particularly vulnerable to some variable that is unforeseen, causing huge fluctuations in the food supply? We don't know, and nor do we have sufficient understanding of ecosystems and the complex interactions between organisms below and above the ground to possibly create accurate models of how they currently work, much less with a new organism with new genomes in the mix.
There are studies of the impact of specific GMOs on specific variables, such as on a particular pest. For example, a GMO can be designed that would excrete a pesticide on its own leaves rather than needing this pesticide to be sprayed on. Results have shown that this type of pesticide distribution can be more harmful to natural cycles since it has a more extreme effect than standard pesticides do, and cannot be adjusted after it has been produced.
As a side note, GMOs are not designed for extra food production. Traditional breeding is sufficient for this. GMOs are designed to reduce pesticide application, work with other chemical products by the same supplier, and to assert ownership rights over living things. Legally, they are more like a product that is owned by the company producing it. Thus, if a farmer buys GMO seeds, those plants are designed to work with other products from the same company, and to require future purchases by that farmer from the same supplier. If those genes spread via natural reproduction to other farmer's fields, the supplier has sued those farmers for having their product on their field, even though the "product" is a living being which spreads itself. This has actually happened, causing huge fees to neighboring farmers.
To sum up, we don't know the true harm of GMOs to the environment, though we know they present a risk greater than invasive species and traditional breeding. However, there are huge economic and legal impacts of GMOs since these are a new type of "product" that is owned by a private company, yet behaves like a living being, reproducing itself.