Yes, you can install a Shimano 105 or Ultegra road power meter (such as a 4iiii or Stages R7000/7100 or R8000/8100) on Shimano GRX crankset as they both use the same interface – which is called Shimano Hollowtech II.
However, there are two things to note:
1) Most importantly, the GRX crank arm uses a different (wider) offset than the road crank arms. This means your stance will be slightly asymmetrical.
2) The color and the look of the crank arms won’t be exactly the same (a cosmetic issue only), but they are very close so most customers are ok with this.
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.