There are loads of planning tools to create GPX files, these include:
- BaseCamp
- Garmin Explore
- GPSTraining OS Route Planner
- Komoot
- OS App
- Outdoor Active
- Plotaroute
- etc.
The list is very long.
At the end of the day it is a personal choice of features, functionality, ease of use, etc.
What is best for one person may be unusable or overly complicated for another.
Try some out, see which you prefer.
There are also loads of sites that offer pre-walked/hiked track/route files (in GPX format). So, you may be able to find the GPX file has already been created for you. Sites that offer track/route (GPX) files, include:
- Outdoor Active
- Komoot
- AllTrails
- Walk Highlands
- Wikiloc
- GPSTraining
- Talkytoaster
Again, the list is very long. Simply Google the path/trail name with GPX in the search term, e.g. "GR1 GPX" or "Snowdon Pyg GPX", and so on.