Gmail has been a bit problematical of late. The best thing to do is delete the account and re-enter the details making sure you put in the correct password. Gmail is an easy account to set up compared with some others. You are not alone with this problem but as to why it happens, anyone's guess. Lastly, once you have re-entered your account details successfully, reboot your iPad.
Let us know how you get on and good luck.
Mike