Here is a tip for registering multiple Azure AD B2B guest user accounts using the same Gmail address. These multiple accounts (logins) can then be used for different identities in Microsoft 365 i.e. different logins to a SharePoint site.
I written a few low/moderate code Azure AD B2B provisioning solutions using Power Automate (Flow), PowerApps, PowerShell and Microsoft Graph. I often use my gmail account for testing the process.
Gmail ignores the full stop (period) in first part of the email address before the @ symbol.
So if your email address is email@example.com and an email is sent to firstname.lastname@example.org, then the email will still be delivered to email@example.com.
This means that you can have many different B2B (Guest) user accounts in Azure AD that you can use for different guest user accounts in Azure AD by adding/removing full stops in your Gmail address
You could have for example vary the email address firstname.lastname@example.org into email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org and there are many more different combinations.
All the invitation emails even though they are going to variations of your email address will go to your same gmail.com address and you can then sign in using the different variations.