Spread horseradish onto 2 of the 3 slices of white bread.
Warm the turkey and shred it with forks. Place half of the turkey on top of one horseradish-topped slice of bread. Place the other horseradish-topped slice on top of the turkey, and put the remaining turkey onto it. Then, top it all with the plain slice of bread.
Warm the turkey gravy and gently pour it over top of the sandwich. Serve warm.