Were you previously using POP3 or IMAP to access and download your Gmail to your old iPad? This will have significant impact upon where your email messages were/are stored.
If all devices were/are using IMAP to access your email account(s), all devices that access your mail account(s) will remain faithfully synchronised via your Mail Providers mail servers; each device will have a copy of the mail messages that remain stored on the mail server until deleted.
If instead devices were/are using POP3, mail messages may be downloaded (and deleted) from the mail server by the first device that accesses the mailbox. If your older device(s) were using POP3, your archived email may have been downloaded to the iPad and deleted from the Gmail server - and as such may no longer be accessible to your new device.
If some devices are using POP3 and others using IMAP, you see potentially unexpected results. While it is possible to use a mix or IMAP and POP3 clients, this requires considerable care with configuration if issues are to be avoided.
Consistent and reliable management of Mail can be achieved across multiple devices by ensuring that all devices are using IMAP. If you must use POP3, you must ensure that POP3 is configured on each device to "download and leave a copy on the Server" - and not "download and delete".
Another potential issue is how you authenticate to access your Gmail account. Google have, until relatively recently, used App Specific Passwords - but have now changed their authentication protocols to require the use of OAuth. Simply deleting and re-adding your Gmail account using the Account Wizard should apply the correct authentication method for your Gmail account.
Settings > Mail > Accounts > [Your GMail Account] > Delete Account
Force restart iPad - Apple Support
Settings > Mail > Add Account
NB: For iOS/iPadOS 18.x, the path to Mail settings is slightly different:
Settings > Apps > Mail > Mail Accounts >
After deleting the account, perform a forced restart before re-adding the email account to your iPad.
