What is Gmail error 403 disallowed_useragent?
You cannot sign in from a third-party app because it doesn’t comply with Goolgel’s embedded webview policy. This might be due to the unsafe algorithm of that particular app that Google doesn’t allow users to use for Gmail.
You can contact the developer to resolve this issue. However, you should also understand the above reason why Google doesn’t want you to use third party app to open Gmail. Embedded WebViews can put security at risk because they let third-party access and change communications between the user and Google.
Fix Gmail error 403 disallowed_useragent
Go to Google Play Store to download and install Chrome. Now set Chrome as the default browser and add your Gmail account. This is the only way to proceed without any issue.
In case you want to continue with the third party app. You should contact the developer and ask for the changes to fix Gmail 403 disallowed_useragent issue.