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

Video below shows how to change directory for offline maps to SD card on Android 5.0 as on higher versions this process is simpler. Unfortunately it is not possible to change directory for offline maps to SD card on devices with Android 4.4.4 and lower. On Android 6.0 and higher when you navigate to SD card you just need to grant permission to write to SD card through showed dialog. On Android 5.0 in showed dialog you need to navigate to actual SD card into which you want to store offline maps and then tap on Select all button. You can also follow step by step tutorial for Android 5.0.

Below are step by step instructions how to change directory for offline maps to SD card.

1. First off you need to start GPX Viewer PRO.
2. In top toolbar tap on map button without globe. If you have 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 maps manager, when you do this you can continue to step 4 of this how to.
3. In Offline map dialog tap on Download button at bottom.
4. In Offline maps manager tap on … button in top right corner.
5. From menu select Change offline maps directory.
6. In Android 6.0 and higher you will be asked to grant GPX Viewer PRO permission to read and write external storage if you haven't granted it yet. In order for GPX Viewer PRO to change directory for offline maps you need to grant this permission to GPX Viewer PRO.
7. In showed dialog navigate to SD card directory (basically it has path /storage/dir where dir may be sdcard0, sdcard1 or some not trivial name like 81AL-65R0, this depends on device and Android version). In Android 6.0 and higher when opening SD card directory you will be asked to grant GPX Viewer PRO permission to read and write to SD card if you haven't granted it yet. In order for GPX Viewer PRO to change directory for offline maps you need to grant this permission to GPX Viewer PRO.
8. Then tap on + button in top right corner to create directory, give it a name and tap on OK button.
9. After this on Android 5.0 it will show screen with storage selection. In left showed menu tap on storage which is your SD card and after this at bottom tap on Select all button. After this tap on Confirm button. You can review this step in video below.

Below is video showing following instructions above on Android 5.0: