It's underneath a half-destroyed highway bridge, and can't be missed (though it's only made up of a pair of shanties). Girdershade isn't really a difficult place to find in fact, if you instantly teleport to Smith Casey's Garage (which you'll have to visit mandatorily in order to access the main quest branch, Tranquility Lane), you can travel southwest from that location and run right into Girdershade.
If you want to undertake a quest entitled The Nuka-Cola Challenge, then you're going to have to head to a tiny, tiny settlement in the southwest corner of the Capital Wasteland called Girdershade.
