If you are looking to change the Password for the Gmail email account itself, you’ll need to sign-in to Google using a web-browser and change the password there.
After changing the account password, the required authentication to access the account from your iPad Mail App must be updated. The corresponding account on your iPad may be configured in a number of different ways - and may use either an App-specific Password (if the account was created manually on your iPad) or OAuth 2.0 Token.
You can determine the configured GMail authentication method here:
Settings > Mail > [Gmail Account] > Account > SMTP > Gmail SMTP Server > Authentication
If the account is configured to use OAuth Token, you’ll not be able to edit this parameter; if configured to use Password, it can be changed. Be aware that for Gmail this is not your Google Account password - but is instead an App-specific password that must be generated by Google Mail for your device. This password is generated (using a web-browser) from the Google Account page. If using Password authentication, you will also need to update password for the Incoming Server on your iPad.
This Google Support page details the OAuth Token revocation and reauthorisation process that occurs after a change of account password:
https://support.google.com/a/answer/6328616?hl=en
I hope this explanation and guidance proves to be helpful.