Answer: Gaura is sometimes included on lists of deer resistant plants, however if they are hungry enough they will still eat it. Also, your local deer population may have learned to eat different plants, which may or may not include it so far. They learn to eat new plants when browse pressure is severe, they are very adaptable. (This is true of most deer resistant plants.)
In a quick check, I did not find Gaura listed as toxic to livestock/horses, however most of the toxicity lists are primarily oriented toward reported incidents involving landscape or common trees and shrubs rather than perennials or herbaceous native plants. For this reason I would strongly suggest you please consult with a veterinarian who specializes in horses to find out if they would eat it, and if it would be safe if they did.
I'm sorry I can't be more specific for you.
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