GMOs aren't really added directly to the meat, beef. However, beef cattle may consume feed that comes from a genetically modified plant. All beef cattle begin their lives on a farm or ranch, grazing pasture or grass - none of which is considered a GMO. For many cows this will be their sole source of feed for their lifetime. Some cattle receive rations of grain, which may contain corn or soybeans, both of which have genetically modified hybrids and varieties. Cattle will also eat alfalfa hay. Alfalfa is another crop with a genetically modified option.
As an example, on my parents' farm, our cows and calves will graze on pasture from May through November. In November the calves are weaned and taken to the lot where they will be fed grass and alfalfa hay as well as a grain ration. In December the cows are brought to their winter lot. Throughout the winter, they will eat grass/alfalfa hay and grain. The alfalfa we feed is considered non-GMO, but the grains probably are, since we grow those on our farm and have chosen to plant mostly genetically modified seed.