I have searched all over and cannot find the crash site for the Alien Blaster or the Mercs who have the Gauss Rifle.
Anybody help me please?

Take the road leading noth west from New Vegas, leads to crater. Along way you will find it.

It's near the Tumbleweed Farm. Walk north and you'll run into something like a rock face that makes the northern boundary of the map. Then head east along the rock wall, and you should run into the mercs OR the aliens. You'll only get the aliens if you have Wild Wasteland, IIRC. Otherwise, you get the mercs.

Thanks for that it was driving me nuts. Found the Mercs with the gauss rifle, lovely bit of kit.!!!!!