You can use the brass and yes the primer is the issue. I have never bothered and gave my M2 brass away, perhaps another can comment on how do deprime. If I recall correctly a standard depriming pin won't work. Then, you'll need a special tool to round out the primer pocket.
Remember that you cannot interchange loads worked up in military and commercial brass. The military brass is thicker. Also, make sure you read up on how to properly reload for the Garand, especially regarding proper sizing and powder selection. That is one gun you can get into trouble with if you do not do it right. You have to have the right pressure at the gas vent or you can bend the operating rod and shatter the stock.