I first saw G.S.W. left foot when reading a record about a disabled Mexican War vet. He was wounded in 1847 at Buena Vista and discharged in 1850 for disability.
He was admitted to the U S National Home for disabled Vol Soldiers in Sawtelle CA in 1898.
I searched all the old medical terms I could find but GSW was not listed. I then googled “GSW left foot” and a lot of transcriptions came up with that term and different parts of the body. Apparently it was in common use at one time.