Backcrossing/“breeding” is relatively uncontroversial and not considered GMO — though it may be controversial whether the eventual derived breed is a true American Chestnut.

There is a GMO effort which adds a gene to the American Chestnut so that it produces an enzyme called oxalate oxidase; the gene comes from wheat plants. The enzyme breaks down the oxalate acid produced by the blight fungus. The first GMO tree was planted in 2006 and has held up, as have others in the test plot. IIRC, USDA permission is being sought so it can be planted freely. Assuming it wins approval, I’m really hoping I can get some seedlings in a few years to plant in my forest.

https://www.esf.edu/chestnut/genes.htm

"To save the American chestnut tree, researchers want to release genetically engineered trees into the wild to reproduce. It would be a first — a possible breakthrough and an irreversible experiment."

https://acf.org/ny/news-events/how-gmos-might-save-the-ameri...