There is a long thread on getting GMS engines running properly in the glow engine forum.
To save you a lot of reading, the bottom line is that with GMS engines, you need to drill through the fuel inlet. Take the carb off the motor, and using a 1.5 mm drill (1/16" is close enough) drill through the fuel nipple into the brass bit where the fuel gets metered by the needle valve - but try not to go through the other side. When the GMS engines are assembled, the holes for the fuel don't always line up. Drilling through the inlet nipple takes care of this mis-match. The problem usually shows up when running at WOT and the carb is not able to supply enough fuel.
I have several GMS engines (including the .47 like yours) and I have done this on each one and never had a problem. My .47 used to always cut out on me until I drilled it. Never had a dead stick since.
But, in your case, it may be something different, as it is cutting out while taxiing, not at WOT. I was going to say replace the GMS plug, as they are rubbish, but I see you are using a #8, which is good.
Another thing you can do is to take out one of the head shims. GMS engines have quite low compression as supplied. There are 3 shims under the head - 1 thick and 2 thin. Try taking out one of the thin ones and see if it improves things.
I now run my GMS .47 on 0% nitro with only the thick shim in place and it screams, and idles just as well as when I ran it on 15% nitro.