Illinois Civil War Muster and Descriptive Rolls Database (Illinois State Archives):
      Wiel, Gustoff - Pvt - Co B - 12 IL US INF
       Join, 22Apr1861-Mendota; Age 25
       Mustered out, 01Aug1861
American Civil War Soldiers (ancestry.com)
     
Civil War Soldiers & Sailors System Search Detail (National Park Service):
      Gustoff Weil  - Co B - 12 Illinois Infantry (3months, 1861)
      Soldier's Rank in - Pvt
      Soldier's Rank out - Pvt
      Film Number M539, Roll 96
Honor Roll State of Illinois, Illinois Veteran's Commission, Oct 1, 1956:
     Weil, Gustaf - Pvt - Co A - 12 IL INF
      Died, 18Jul1910; Buried, Elmhurst Cemetery, Joliet, Will Co, IL - Ridgeland Section
National Graves Registration Database (Sons of Union Veterans of the Civil War):
        Weil, Gustaf  - 12 Illinois Infantry
         Died, 18Jul1910; Buried, Elmhurst [Joliet] Will Co, IL
Will County Illinois USGenWeb Necrologist Reports (© 2002 The ILGenWeb Project All Rights Reserved):

Headstone Readings, Elmhurst Cemetery, Joliet, Will Co., IL - Ridgeland Section:
      (Large "Weil" family stone - separate makers)

FATHER Gustav Weil 1835-1910

MOTHER Margaret 1844-1922

SON Gustav Weil 1878-1937

SON William Weil 1866-1931

        (G.A.R. Marker)

DAUGHTER Matilda Weil 1870-1966

DAUGHTER Stella M. Weil 1885-1952

DAUGHTERS Ida Weil        Julia Weil             1880          1887-1887