I've done this kind of thing but it's not too reliable. It's not done internal to the GPS either.
'Mark' your position and take a 'sight and go' line from that point then 'project' the waypoint. You'll have to guess the distance. I usually go way over and say '30 miles'
Do the same again from another point; The further away from the original the better. Ideally a few miles or more. - 100 yards would be useless. Then load all those 'marked' and projected points into, say, Memory Map. Join the line pairs with a route and Voila! Where the lines intersect should be near to what you were trying to identify. Unless, you didn't guess at a large enough distance for the projection...
I certainly couldn't recommend this as a navigation method.