If this is the case, there is nothing GoodTask can do. GoodTask can only speak to Reminders, not Outlook.
When you add a task on GoodTask, it tells Reminders to add it. If it goes to Reminders, GoodTask has done it's job. Reminders needs to send it to Outlook. That's why I asked you to check connection between Reminders and Outlook in default Settings app.
You can go into default Settings app and go into 'Passwords & Accounts'. Check your account there and turn off 'Reminders' and turn it back on.
I've tested adding on GoodTask for Outlook lists and it seems to work fine.
Thanks!