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 Map Menu
2. Tap the circle icon with the three dots
3. Tap on 'Overlays Menu'
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 to exit the Layers menu and return to the map menu.