Is there an app for a smartphone that operates like a GPS that can aid in marking tree stand locations. I currently use a Garmin which works great but would like to reduce equipment. I general travel to and from my stands in darkness.

Absolutely!  We all use the Hunstand app to do exactly what you're talking about.  You can mark stand locations, camera locations, trails, property boundaries, measure food plots and a whole lot more.  You can also share it all with your hunting buddies if you want.  It's the best app we've found for hunting.