You will first have to send the gpx file to your iPhone. The easiest way is to use mail:
Send yourself an email with the gpx file attached.
Open de mail on your iPhone.
Tap and hold for longer than a second the attachment icon in your mail.
When you tap shortly the content of the gpx file will be shown (a lot of gibberish): in this case tap the Open in…/Share icon = upper right button.
Select Pocket Earth to open the gpx file with… Voila!
You can view the file right after the import. You will also find it listed in the Favorites or My Tracks.
Instead of the mail I recommend to use a cloud drive like Dropbox or Google Drive. When you place all your gpx files in your cloud drive on your computer/laptop you will have them automatically available on your iPhone (and all other devices) with the cloud drive app (i.e Dropbox app). When your collection of gpx files grows this is the best way of administrating them. By opening the cloud drive App (dropbox or google drive) and tapping the gpx file you can open them in the same way as using the mail app.