Offline map is available only in GPX Viewer PRO. So this how to won't work in free version of GPX Viewer.

GPX Viewer PRO supports adding custom offline map data stored in MBTiles (SQLite database) files. These MBTiles must contain offline map data in raster format (images, for example png or jpeg).

Below are step by step instructions how to add custom offline map data stored in MBTiles.

1. First off you need to start GPX Viewer PRO.
2. In top toolbar tap on map button without globe. If you hidden this button previously tap on … button in top right corner in this top toolbar and from menu select Offline map. If you have also hidden this button, open left main menu, then tap on Settings button. In settings screen tap on Maps button and then Offline map data manager, when you do this you can continue to step 4 of this how to.
3. In Offline map dialog tap on Raster map item. Then tap on paint button to the right of Raster map item.
4. In Offline map style tap on MBTiles file with raster offline map data item, then in file browser find and select your custom offline map data stored in MBTiles file.
5. Tap on back button.
6. Tap on switch button in top right corner of Offline map dialog to enable this switch. Then tap on OK button.
7. GPX Viewer PRO will switch to offline map and show it shortly on the screen. If you have blank screen instead try zooming out to world level zoom and then try zooming in.

Below is video showing following instructions above: