Here are the power meters that you can consider for your SRAM GXP road crankset:
Left Crank Arm Power Meter
Stages Cycling makes the Stages Carbon GXP for SRAM Road Power Meter which is compatible SRAM GXP road cranksets. This power meter replaces your left side crank arm and measure left leg power.

Stages Carbon GXP for SRAM Road Power Meter
Please note, whenever you are installing a left side power meter crank arm, you need to confirm you have 10mm of clearance between the left crank arm and the chainstay in order for the sensor to clear. Please see the image below.
Pros: Easy to install, don’t have to change crankset
Cons: Measures left leg power only
Spider Power Meters
power2max NGeco SRAM Road Power Meter

power2max NGeco SRAM Road Power Meter
If your SRAM crankset uses the 3-bolt interface (image below), you can also use the power2max. This power meter measures power (both legs) at the crank spider. power2max makes some of the most reliable power meters we sell.
SRAM RED/Force AXS 107 BCD Power Meter Spider

SRAM RED/Force AXS 107 BCD Power Meter Spider
Some newer SRAM Force GXP cranksets feature an 8-bolt interface. If this is the crankset you have, we would recommend the Force AXS 107 Power Meter Spider. Like the power2max above, this is a spider-based power meter that will bolt right onto your crankset and will measure power from both legs. Please note, this power meter features a 107 BCD and is indented for smaller chainrings (gravel).
Pros: Moderately priced, VERY reliable, don’t have to change crankset, measures both legs
Cons: None
Power Meter Pedals
Power meter pedals can be installed on any crankset. They are reliable, accurate, easy to install and can be moved between bikes. We carry every power meter pedal available including ones from Favero, Garmin, SRM/LOOK and Wahoo. This includes both single- and dual-sided versions as well as pedals for SPD-SL, LOOK Keo and Speedplay cleats.
Pros: Easy to install, works on any bike, can move from bike to bike, don’t have to change crankset
Cons: None