This is very easy gun to abtain and all you need to do is purchase it from "Ronnie Shaw" at "the castle" now to gain access or to even open up the castle you must have done a few quests for the minutemen, after arriving at the castle you must clear out the area and Ronnie will arrive, afterwards you are free to purchase the gun from her. This gun is near 5000 caps but you can also buy non unique gauss rifles from her aswell for cheaper.

Gauss rifle, 50% more limb damage

Here is where the castle is on your map
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Here is a video