how accurate are gaffrig redline speedometers

Not very, unless you use a carefully setup pitot assy in a clean water position (this is hard to find on a step bottom).

No real reason to spend big bucks on a pitot style speedo. GPS speedo is far more accurate, and usually not much more $$. GPS speedos don't respond as quickly on accel, but they are more accurate.

A paddlewheel tournament speedo is very accurate up to about 50 mph. Above that, they lose accuracy due to dirty water.

how does my speedo get its reading? there isn't a visible sensor by the drives

If you are using a pitot tube speedometer, it will get its pressure reading from either a separately mounted pitot assembly OR from a pitot hole in the gearcase of your lower unit (assuming you have a standard Bravo or Alpha drive made since 1990). The drive pitot hole will be appx 3" above the propshaft centerline. They work well on installations with a low x dimension and a nonstepped hull.

i have a 2003 292 fastech with twin 6.2 320hp mercs. when i flush the engines i notice that the port side prop very slightly spins is this normal?

I have a 1999 271 Fastech. Gaffrig speedo and GPS readings are within 1 mph of each other up to 72 mph. I understand my case is an exception.....not a rule.