Finally I've solved the problem and hope my experience could help someone else who is banging his/her head on this scenario.
At time of writing I was accessing the GCP cluster via public IP addresses.
I had a /etc/hosts file resolving hostnames to their public IP
Then customer's NOC granted me access from internal network over VPN and next attempt went fine.
Still can't understand why, maybe some networking related config issue, but being on the internal side of the network solved my problem
Simple as that!
Thanks for your precious support, and patience
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