This article explains how to activate grid line overlays in Gaia GPS for the iOS app. You can choose between Lat/Lon, UTM, MGRS, and the Distance grid overlay.
Add a Grid Overlay to the Map
1. Open the Layers Menu
2. Tap on 'Overlays'
3. Tap on 'Grid Overlay' to access the grid line options. Select any of the grid overlay options to use on the main map screen.
4. When you are finished, tap on the back arrow and then tap the 'x' to exit the Layers menu and return to the main map.