I sign-up using my Gmail account. But there is no Gmail account login for the Apple app.
the new native iPhone app which we'll release within the next days will have such functionality, however if you're using the current app with a Todoist account created with a Google Account, you'll need to set a separate password for your Todoist account. To do so, go to Todoist Preferences (gear icon in the upper-right corner) -> Account and add a password there.
Note - you will still be able to log into the web-app with one click using your Google Account, but for the purpose of logging into a mobile app, please use your GMal address in combination with the newly created password.