Red Gate is sort of hidden. There's so many location markers on your pip-boy it just gets covered up. Try and get to the upper area, on top of the rocks.
Yes, I finally found it. I started from Red Rapids and went North West. If you zoom all the way into the map, Red Rapids is in the bottom right corner of the map square. Red Gate is at the top of the same square, about 1/3 of the way in from the right. You have to climb. The map marker will not show up until you actually discover it.
At the top is a skeleton against a wall with a duffel bag. The Survivalist Rifle is in that bag.