If you have downloaded GoodNotes 5 as a free version or already upgraded to the full version on one device and you would like to use it on another, you can simply redownload GoodNotes 5 from the App Store or your Purchase History. As long as your other Apple devices (iPads, iPhones, Macs) are logged in with the same Apple ID you used for the original GoodNotes 5 purchase/download, you will be able to use the app on the other devices.
If you are a paying user and experience difficulties in unlocking the full app on second device, see here: I can't restore my purchase