I'm not surprised about Rother Verlag, but their guides are really popular in german language countries. I have downloaded the files, because I own some of the books, but the GPX files themselves are not and have never been exported to OSM, quite often they are quite inaccurate, some meters away, and with some tours you get really in troubles. But you can find the trails they recommend (usually some connected together, or they suggest routes without any existing trails) when looking at OSM or OS maps (yes I paid to access them, too), so the paths themselves are no way copyright protected. But at least I think we should remove all RWxx numbers from OSM, as they are really private and marked not on the spot.
It would be great if you could convince the Walkhighlands founders to allow access to their GPX trails or simply their hiking trail proposals to OSM. I'm happy to study their proposals and create OSM relations for the access routes to some popular Munros and other hills and mention walkhighlands as a source in OSM. And even more to link from Wikivoyage to somewhere with a open license access, or maybe even their website, if the tracks will remain available for some time. You see the examples in Wikipedia, where many of the webresources in citations have disappeared for years. The bigger task is to create Pages for the places in WV and compile the information, in the german language section, we adress people who are not that well english speaking. Even for me the british hiking books are sometimes challenging because some words are not commonly used in English taught at school in our country. So Travellers might get stuck when reading "follow the corrie and climb over the stile...", but thats similar in Italy and Greece, all have their own expression for trail elemants and landmarks... regards Martin