Gauss Rifles are essentially nonexistent in Brotherhood of Steel ranks, except during a few quests.  That isn't really very realistic, because the Brotherhood is the biggest hoarder of technological weapons including Gauss Rifles.  Now, you can find them being used on occasion by Brotherhood Knights and Paladins who would otherwise be using Miniguns or Gating Lasers (which can still happen, now each of them have a 1/3 chance of spawning).