This article explains how to activate grid line overlays in Gaia GPS for iOS
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. Tap on the back arrow then tap the 'x' to exit the Layers menu and return to the main map.