They are hardy in the ground for me so they probably will be for you too, wallaby. I love them. I think a lot of my old ones got dug/chopped up when I kept adding more lilies. Well I had to plant them somewhere!
Back in the late 1970's, I grew some Gladiolus nanus here in zone 4. But they came from a place like Burpee's or Jung's, so I can't say if that is actually what they are. Anyway, they pretty much all returned after the first winter, most did not after the second, and the few that did make it died the following winter.
The area however, was not the best place for glads that might be winter hardy. It was in a valley (we had 3.5 acres[1.4 hectares]) where water would sit for 1-3 days each spring. On the other hand, it was the same garden where a no name red glad (that I spoke of in a previous thread somewhere here) survived and multiplied five winters without fail.