Not sure what a Gremlin is. I never seen one of those before so hard to say what size prop.

usually high speed props are in smaller diameter with high pitch. But it all depends on the type of plane also and weight. You can go to where the prop gets to small and has no thrust to tote it around.

If you go down in diameter go up it pitch. So if you was using 9x6 I would go down to a 8x7 or maybe 8x8 or 7x8. I am not sure if 8x8 is to much so just guessing here. These are just ballpark figures.

Experiment some with different props. You need to get as much RPM as possible to go fast. So thats where the smaller size props come in to turn fast. Then you find a good pitch that can turn fast with that certain size diameter. That should get you moving out pretty good.