Have you logged on to gmail webmail account using a browser and selected the type of forwarding - POP or IMAP? If no then you need to select which option you want to use and then you have to use that Server type when creating a mail account in thunderbird. Is this a POP or IMAP mail account ? If POP have you logged onto your gmail webmail account and selected to use 'Less secure apps' ? If no, then suggest you do it. Do you use a phone to access gmail account? If yes, you use webmail, phone, other computers/devices to access the account, then have you set up 2 step verification to get access? If yes, I have two step verification, then have you generated an 'application specific password' to use in Thunderbird? If you use IMAP, is your 'Authentication method' set to use OAuth2? If using imap and set to use OAuth2, you do not need to select 'less secure apps'.