You have a couple of great gravel power meter options for a Shimano GRX crankset:
Left Crank Arm Power Meter
Both 4iiii and Stages make a Shimano GRX RX810 power meter which is compatible with all Shimano GRX cranksets including GRX RX600. With this type of power meter, you swap out your current left crank arm for the new power-equipped version. These power meters are affordable and easy to install. The trade-off is they only measure left leg power.
Please note, these power meters attach a sensor to the inside of the left crank arm. On some bikes, there is not enough room between the left crank arm and the chainstay for the sensor to clear the frame. Therefore, you should make sure you have at least 10 mm of clearance for Stages and 4iiii PRECISION 2 power meters and 8 mm of clearance for 4iiii PRECISION 3 power meters. Also, please make sure that you don’t have a bottom bracket mounted rear brake (used on some aero and TT bikes). Please refer to the image below.
Pros: Affordable, easy to install, don’t have to change crankset
Cons: Measures left leg power only
Spider Power Meter
This power2max power meter is a direct replacement for your Shimano GRX crankset. The power2max Gravel Power Meter Crankset uses the same 24MM spindle as your GRX crankset, so this power meter fits your current bottom bracket without any changes. This makes installation very easy. Even better, the power meter gives you a narrow road Q factor vs. the slightly wider Q factor inherent in GRX cranks and allows you to run a 1x or 2x chainring set-up.
Pros: Moderately priced, VERY reliable, measures both legs
Power Meter Pedals
MTB power meter pedals can be installed on any gravel crankset. We have both the Garmin Rally XC and the SRM X-Power.
Pros: Easy to install, works on any bike, can move from bike to bike, don’t have to change crankset