You cannot determine the towing capacity without knowing the specific model (1500, 2500HD, 3500HD) and reviewing the sticker located inside the driver's side doorjamb. This sticker will list the GVWR (Gross Vehicle Weight Rating), GAWR (Gross Axle Weight Rating), and the maximum towing capacity for *that specific truck*.
Looking up the specs online for a general 2005 Chevy Duramax 6.6L will give you a range, but it won't be the precise number for your truck. Always check the doorjamb sticker.