Due to his age, Granville Dennis was not subject to Confederate conscription. That's why we find him on a parish militia rolls rather than with a Confederate military command.

You asked about the occupation of the Tennessee Valley. The 1860 census of Morgan County AL reports Granville Dennis and family living near Decatur, a town seized by Federal troops on April 13, 1862. Counterattacks by Col. John Scott's 1st Louisiana Cavalry provoked Federal officers, who allowed their troops to ransack the town of Athens in retaliation. If the Dennis family was "burned out" of Morgan County, it would have occured during the spring or summer of 1862. Here are articles concerning the Federal officers responsible for the occupation of the Tennessee Valley and execution of U.S. military policy towards civilians found in that area.

http://www.thelatinlibrary.com/chron/civilwarnotes/mitchel.html

http://encyclopediaofalabama.org/face/Article.jsp?id=h-1652

Wilson certainly is a common last name. If we limit our search to first name George, last name Wilson, 1,259 index references to Civil War service files will appear. I only wanted to mention another George W. Wilson who lived in Walker County AL.

The index to all service files is posted below, your Wilsons appearing as members of Co. "M", 1st Alabama (U.S.) Cavalry. The entries Wilson, Union and Alabama will be sufficient to display their names and service unit designations.

http://www.civilwar.nps.gov/cwss/soldiers.cfm

If you don't have actual copies of their military service files, they can be ordered here.