Gladiolus is a large genus, with scores of species and many more hybrids. Most of the commonly grown hybrids need to be dug up and stored dry for the winter, but some of them are cold hardy. The best way to find such hardy glads is to observe gardens in places where winters are harsh, since many gardeners plant gladioli and then don't dig them up. The survivors will multiply and thrive in these gardens.
This glad is one I found in a garden in Bluefield, West Virginia, right across the street from my uncle and aunt's house. I could tell by the presence of many young plants, in overcrowded clumps, that they had been left outside for several winters. The owner confirmed this, and kindly allowed me to take a small clump of plants. Bluefield, also known as "nature's air conditioned city" is a small city at a high elevation in extreme southern WV, right off I-77, (there is also a smaller town by the same name right next to it in Va) with a relatively cool, pleasant summer climate and pretty rough winters for a southerly location. While warm spells are more frequent in winter than here in southern NY, cold spells are a lot more severe than here in coastal NY, with lows below zero F not uncommon. I have observed some plants in Bluefield that are also considered marginal here, but survive, such as Magnolia grandiflora and Crepe Myrtle. This year the Crepe Myrtle plant in my relative's yard was killed back to the ground by the cold this last winter, whereas those in my town in NY did not suffer such damage.
I have no idea which gladiolus cultivar this plant is, but hopefully it will like its new home in NY.