The gas tube length you will need will be a mid length gas tube.
The Wilson 20" 308 caliber barrel will require the Wilson 15.5" rifle gas tube.
Not the standard rifle gas tube which is 15.125" commonly used with other manufactures.
The 15.5" gas tube will not work with standard rifles either. It will not allow the BCG to fully engage.
Gas block size is 0.875"
The gas block required is a .875" and the required gas tube is a rifle length.