Sorry,
I don't know how to use gpx. Although I can talk you through the route! I live at the foot of the Brecon Beacons! Are you using a Road Bike or MTB? I've ridden from Brecon to Cardiff and return a few times! When using a road bike you can use road and Taff Trail for most of the route; then A road or B road for the final 17, miles. Depending on your direction once you get above Merthyr Tydfil? Three routes to choose from.
Using an MTB it's possible to use the Taff Trail for 80% of the ride from Cardiff To Brecon, which would need to incorporate The Old Roman Road Gap (a rough ride)
Sorry I cannot help with GPX, It may not be of any use to you in some places on the ride?
M