Greetings:
My GMS is nearly broken in after 5 flights. It has been turning a 13x5 prop at 12300 rpm and until you mentioned the engine have very little compression, I had't noticed.
When I immediately checked, I found that it doesn't seem to have much compression, but having just flown it wide open yesterday and taching her, I can confidently say, it is not a problem. For the $80 spent, it is a fine engine in my eight-pound Chipmunk. Flies the verticals, and doesn't require a lot of tinkering.