It took me more than two weeks to get to this and I'm no cartographer! These are the limits of the region of Gévaudan, according to the data from the maps "by Cassini.
The first one shows the main roads and the main cities.
The second is a map of the province put on a map from today. There must be some slightly diffrences between the "real" map and I did because I did it at a too small scale, anyway it's wrong to say that the province "was located where today we find the Lozère". The county of Saugues was included, as well as two protrusions in the south of Rouergue and in the east of Vivarais.