I close an old Microsoft account that was created with a Gmail address, will my Gmail account get deleted too?

Closing a Microsoft account that uses a Gmail address does not delete or affect the underlying Gmail account.

The Microsoft account is separate from the Google account; closing the Microsoft account only removes Microsoft data and access (for example Outlook.com, OneDrive, Xbox, etc.) associated with that Microsoft account. The Gmail mailbox and Google services remain managed by Google and are not removed by Microsoft.

When a Microsoft account is closed and the reopen window has passed, Microsoft deletes the data and content stored with that Microsoft account, not the external email provider’s account.

References:

• Close your Outlook.com account
• How to close your Microsoft account