@sns
1.Here’re steps to enable external sharing.
(1)Go to SharePoint admin center -> Polices -> Sharing -> Set "Content can be shared with" as "Anyone".
(2)Go to SharePoint admin center -> Active sites -> Select the site -> Sharing -> Set "Site content can be shared with" as "Anyone".
(3)Go to the site -> Site permission -> Site sharing.
2.By default, Gmail account is allowed. If you encounter issues, please provide screenshots of share to specific user with Gmail account.
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