ORIGINAL: RevGQ
Not familiar with the GMS 1.20, but I love my .76. Are you able to swing a 16-18 inch diameter prop? I think the cowl of the Gee Bee will demand at least a 16 in dia. prop, particularly in landing.
On my initial engine test run in my driveway I used an 18-6 Master Airscrew Classic prop. The engine turned it but it appeared to be loading the engine down too much. Getting a needle was more difficult, but I was able to get a decent 4-stroke needle and then find peak RPM. I didn't tach it, but it didn't sound like it was turning much above 7000 RPM.
I test flew with a Master Airscrew 16-8 Wood Simitar prop. The engine didn't have any trouble spinning it. I broke that prop on the prototypical Gee Bee 3 point landing. The next flight will be with an APC 17-6. I'll also probably try an 18-6 wood prop.
If the weather improves as forecast I'll fly tommorow, possibly Sunday if the wind stays below 15mph.
Rich