Q: Changing gmail password

You don't use iOS Mail to change password, if that's what you're trying to do. Use Safari to sign in to your gmail. Change your password there. If you want to enter new password information but unable to find password field, remove gmail account and add it back in.

If you have recently changed the PW for your Gmail account and need to update it in settings on your iPad you will need to remove the account and then re add it.

Add an email account to your iPhone, iPad, or iPod touch - Apple Support