Garden level...and the key word is “guaranteed” not to have been tinkered with. Years ago Texas A&M grew a maroon carrot. I’m afraid of what is being put in engineered food.
There are quite a few that sell non-hybrid, non-GMO seed. The only issue you may have is on wind blown crops that GMO pollen tends to drift and cross.
Click on my handle and go to my about page, I’ve got a list of ones I use. I like Fedco, Jung, Gourmet, Landreth, and Baker Creek especially.
There is a difference between genetically modified foods and those that are selectively bred. As I understand it, those carrots were bred by taking ones that naturally had maroon colors in them and mating them with similar ones. As an Aggie, I was given a mess of them, and they were quite good. they were higher in beta carotine than regular carrots too.