The Gastlosen is a small mountain range in the Western part of Switzerland situated between the Cantons of Bern, Fribourg, and Vaud. Distance wise it is close to the famous villages of Gruyères and Gstaad. By public transportation it is about 1 hr and 40 min from Laussane or Spiez depending on which train and bus connections you can find. Although it is accessible by public transportation it is an area I'd recommend using a car for traveling. It will be easier and faster to go by car than bus for this area, and a car will give you the freedom to really explore the beautiful countryside in the surrounding area. 

    When I went hiking in this area I did the "Gastlosen Tour" hike which is a circular route from the village of Jaun, and at which there is also a cable car you can use to cut out part of the elevation gain. 

    The hike itself is straightforward, and not very difficult.  did the trail counter clockwise which I think might have been a better way to do it just based on how the views present themselves, but I don't think it's hugely important. There are a fair number of people on the trails in the area, but nothing obscene. There are three nice mountain restaurants on the trail that you can grab refreshments at and enjoy the view. This map may give you a better idea of the trail, and of all the hiking options in the surrounding area. This map also demonstrates the sheer number of hiking options in the area. Although the mountains aren't gigantic the mix of the green hills with the large stone spires gave the area a unique charm. If it wasn't so far from Zurich I would definitely have tried to go back to do some of the other trails. This is another reason I think I car might be useful as it will allow you to easily zip around to different trail heads and explore more of the area. 

    I don't have much else to say about the hike, it should be straightforward if you follow the maps, and I can't speak to personal experience on the area because it was just too far to travel to consistently. As a day trip I think that this would be an area best explored as part of a vacation around Laussanne or maybe from Interlaken. However, if you really like this type of landscape I believe it could easily be turned into a trip of its own, and that it might be worth finding a place to spend the night in the area so you can really get from Gstaad to Gruyères and the areas in between.