GPX Viewer PRO supports starting/pausing, stopping, and adding waypoints by automation with Tasker app. This how to will explain how this could be automated with the Tasker app:

1. In Tasker add new task and add new action System → Send Intent with these parameters:

Send Intent [ 
  Action: android.intent.action.RUN
  Cat: None
  Mime Type:
  Data:
  Extra: android.intent.extra.TEXT:TRACK_RECORDING_ACTION_START_PAUSE
  Extra:
  Extra:
  Package: com.vecturagames.android.app.gpxviewer.pro
  Class:
  Target: Activity ]

For starting track recording add android.intent.extra.TEXT with parameter TRACK_RECORDING_ACTION_START_PAUSE, for stopping track recording change it to TRACK_RECORDING_ACTION_STOP and for adding waypoint change it to TRACK_RECORDING_ACTION_WAYPOINT.