The first step is to get ready to open the GPS zip file on your mobile device.
You are going to need a program for unzipping the file so you can access the individual GPX files. I believe Android devices have something already built in. If your device does not seem to have something like that, you will need to download one from the Play store. Obviously I can’t give detailed instructions how to use an app when I don’t know which one you’ll end up with.

Then you will need the Gaia GPS app
Log into the Play store and do a search for “Gaia GPS”. Once you have it installed, open it, skip the account signup, and step through the quick start tour. Give Gaia access to your device’s location services when prompted. Then go back to your home screen.

The next step is to get the GPS zip file onto your device somehow
I can’t provide detailed instructions for every model or platform. But a common way might be to e-mail the file to yourself from a PC. Or, I can e-mail it to you if you don’t have access to a PC.

Here are some steps for getting the file unzipped and into Gaia from e-mail on your device:

With Android OS:
Open the e-mail with the attached file.
Tap on the file attachment.
Tap the download button. Note the location it tells you that it downloaded the file to.
Open the My Files app.
Go to the location the file was saved to. Example: Device storage -> Download.
In my case, the OS already knew how to work with zip files, so when I tapped the file I had an option to Extract.
Tap to extract the files, or extract them using whatever zip app you have.

Import The Files
Note: Be VERY careful with this, a single import brings in just a few tracks, but also brings in dozens of waypoints.
If you import the same file twice you will have duplicates, and deleting hundreds of duplicate waypoints is not easy.
Note: If you sign up for a Gaia supscription you can manage your data using a web page which is easier.

Open Gaia GPS and Start Importing
Tap the icon with the circle and plus in it to bring up the menu.
Tap Import File.
Now you have to find the location of the files. I found mine in sdcard -> Download -> WA_GPSFiles.
Tap the first GPX file. It will say “Importing file” for a moment. When that disappears, wait a few more moments until it says “Successfully imported Track(s)” before proceeding to the next one so the device doesn’t get too far behind.
Note: The “All.gpx” file is too big, and Gaia might not import it.
Note: You can import “Republic_All” and skip Republic_North and Republic_South. Same with Conconully.

Once you are done, zoom out the map in Gaia and you will see the track lines and waypoints on the map.
You can now delete the folder full of GPX files and the zip file from your device.