Google Pay PAN_ONLY payload is not PSD2-compliant and require a separate 3DS flow for SCA compliance if you operate in EEA.
To implement this first you need to submit the encrypted token that you receive from Google in the Create payment (paymentMethods.integrationType=DIRECT) call to Axepta BNP Paribas as usual.
The next steps in the flow should be implemented as described in detail in section 3DS Authentication (Step 4 to step 36).
You can generically implement the 3DS flow for all Google Pay payments. Axepta BNP Paribas will dynamically recongnize PAN_ONLY payloads and start the 3DS process. In case of CRYPTOGRAM_3DS payload, 3DS process will not be initiated.