The Silver Bullet Gauss is an Inner Sphere weapon. The HAG is a Clan weapon.
The SBG also works far less well as a primary weapon than the HAG: it's an excellent crit-seeker but it needs something to punch holes in the enemy. The HAG 40, while not the most efficient at the task, can do an adequate job taking out armor so long as the target doesn't have Hardened, FL, or Anti-Ballistic armor.