I recently changed the password for my gmail account. I was able to register the account on my iPhone but when I do the same on my iPad it tells me that the user id and password are incorrect. I changed the password again but the problem persisted. When I access gmail via Safari or Chrome there is no problem.
Reset the settings on your iPad and then set up everything from scratch. Use the instructions from Apple Support article below. There's no need to change a password to use gmail account on iPad.