Hey @ellsuth !
Unfortunately there are no disc brakes mount on these bikes. You'd have to change the fork, change the wheels, and you'd never be able to put one on the rear wheel ... too much work
But did you know that some Btwin Gravel Bikes were about to get released ? Just a few months to go and the first prototype has been released already : https://www.facebook.com/btwin.bikes/photos/rpp.275099771195/10154616092086196/?type=3&theater