Guard Commander isn't a rank, its a position. E7 is Sergeant First Class, and the Guard Commander position had to be filled by an E6 or higher and they ran the RCF (Regional Correctional Facility, aka prison) I was a guard at. The Guard Commander position would be roughly equivalent with a Warden, but in military prison the Guard Commander would be the senior NCO (Non-Commisioned Officer, generally E5+, with Corporal being the exception as its an NCO but E4 in grade, so you can be an E4 Specialist that isn't an NCO, or an E4 Corporal that is an NCO, but it was vastly more common that anyone passing PLDC (Primary Leadership Development Course, the "school" you have to go to in order to become an NCO) would be promoted to E5 Sergeant at the same time in charge of the prison during that shift, so it was like having a rotating roster of Wardens made from a handful of senior NCOs.