I live an a bison-ranching area, and I have visited several bison ranches and talked to the owners. Here is what I gathered:
The good news is that bison cannot survive the stress of a feedlot, nor will they eat out of a feed trough (because they need to be able to look around while they graze), so they are all raised on grassy pastures.
However, ranchers often put supplemental food and bison "formulas" out on the pasture. The supplements are attractively flavored and packaged so that the bison will eat them. I get the impression that the supplements are more about giving the bison "nutrients" like minerals that might be depleted in the soil, and not a source of caloric food such as corn or soy. So, while there may be no recognizable GMOs in the supplements (no kernels of corn, for example), they could contain synthesized vitamins that could come from GMO-based sources.
If you wanted to be sure, you would have to ask the rancher what gets put out on the pastureland. This might not be possible to trace back to the ranch, but you can try googling the name of the ranch, if it is on the label, and ask them about supplements they put on their pasture.
