There are a few how-to articles around:
https://cloud.google.com/files/CloudVPNGuide-UsingCloudVPNwithAzureVPN.pdf
https://cloud.google.com/solutions/patterns-for-connecting-other-csps-with-gcp?hl=en
Maybe one is helpful.
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten