We can reserve a static IP address on Google following the instructions here.
Would it be possible to migrate this IP address to another Cloud, e.g. Amazon, IBM, or elsewhere, and how would that work.
If this is not possible, how can I have one IP address that I can move around?
The use case is for an SFTP setup, which we have now on Google and is already whitelisted by an external party. I would like to change the SFTP server to be deployed on another Cloud Provider, and to still use this IP address so my client does not have to whitelist another IP address.
Reserving addresses per your link, reserves you a Google-owned IP.
You cannot move Google's IP addresses but you can bring-your-own IP address (blocks) to Google and AWS and possibly others.
The IPs can only be lent to one Cloud provider at a time so you'd have some downtime migrating your IPs from one provider to another.
But, it should be possible (with limitations) to achieve your goal.