For what it's worth, I have the Yamaha owner's manual and service manual for a 95 T9.9. The owner's manual says .020 to .024, the service manual says to gap the plugs at .024 to .028.
You need to post the complete model number from the data plate. The gap can be found in the genuine Yamaha owner's manual or the genuine Yamaha service manual. If your motor uses an NGK CR6HS spark plug, the gap is .024. You need to torque the spark plug when it is installed.