You have an obsolete iPad4 - which cannot be updated to more recent versions of iOS/iPadOS, or receive any security updates. As such, will inevitably encounter difficulties with online services.
Be aware that Google have changed their authentication requirements for Google Mail mail accounts configured for password authentication - and for non-Google mail clients, now requires use of an App Specific password for each mail client/device.
You will need to generate a unique password for your iPad Gmail account - that will differ from your normal Google Account Password. AOL, Yahoo, Google, Microsoft now all use App Specific Passwords when using anything other than their own email client Apps.
If you need to generate an App Specific Password, you’ll need to log-in to your Google Account management webpage - using a web browser - and generate the Password from the appropriate account security page.
More information can be found here:
https://support.google.com/mail/answer/185833?hl=en-GB
While creating your App Specific Password, ensure that your Gmail account is enabled for IMAP; you will find this in your account settings. When correctly configured, you should be able to manually re-add your Gmail account - configured for IMAP - on your iPad4. Google list the correct settings for your device in their FAQs.