I downloaded the android app, and I can't login via gmail.
I login in online via gmail so I am not sure my user or pwd. I can't really even use the android app to connect with my tasks etc.
if you've signed up using your Google Account, you need to set a password in order to access the mobile apps.
To do so, log into the Todoist web-app, click on the gear icon in the top-right corner, choose "Preferences" and go to the "Account" tab where you can set a password for your account.
You will still be able to log into the web-app through your Google account, but will have a separate password which you can use on the mobile site and mobile apps in combination with your GMail address.