Over the years, a number of changes were made to the GAR badge, and these can be used to determine the age range of your find. Originally, on the reverse of the star each point bore a laurel branch. However, in 1886, three of the branches were replaced by emblems representing Sheridan's and Wilson's cavalry, and Hancock's veterans' corps. Yours appears to be one the post-1886 badges.
I found this in the backyard of a Victorian house in Amherst, OH. It was 2 1/2 inches down. This was a veteran's metal........the grave markers were much larger and had a metal rod stuck through them into the ground.