I have the Wild Wasteland perk activated and I got the alien gun, but I still want a gauss rifle. Are there any other locations in the game where I can get one?
From the Wiki, here is a list of rifle locations
Hidden Valley bunker - can be purchased from Knight Torres after following the quest lines. Also, many of the paladins in Brotherhood T-51b power armor have this weapon.
Silver Rush, Freeside - sometimes sold at higher levels.
Carried by Father Elijah
Big MT - can be found on Y-17 trauma override harnesses, most notably near the X-7a "Left Field" artillery launch
Brewer's beer bootlegging - often found in a locked gun cabinet in the cellar. There is a key underneath the Big Book of Science in the other room, otherwise, the locker has a Lockpick skill of 75 needed.