The GMS engines use a muffler that is not very restrictive. Makes for good HP numbers BUT a glow engine uses muffler pressure to help push the fuel to the carb. By reducing the muffler pressure for more performance the pressure to the tank is also reduced making it difficult to get an adequate supply to the carb when the tank is many inches below the carb in a hover. Some engines have better fuel draw and therefore are more forgiving when held in a vertical attitude.

While I do NOT own the GMS 47 I do have the GMS 40 and it can be tuned to sustain a hover but is somewhat sensitive on the needle setting for hovering. There is a sweet spot and it takes some effort to find it. It is very close to what you would use for normal sport tune (ie 300 to 400 rich of peak). Mine is on a Tribute 36 right now and I normally set it just where it turns max rpm when held vertical and then ONE click rich of that setting. Your 47 may require something different, don't give up.