If you're on the free plan of Goodnotes 6 on one device and would like to use the app on another device, you can simply redownload the app from your Purchase History, as long as on your other Apple devices (iPads, iPhones, Macs), you are logged in to the same App Store account you used for the original download.
If you're on the yearly plan or paid for the one-time payment, on other devices please sign in to the same Goodnotes account that you originally created for the subscription/one-time payment.