Android apps can be installed on a Firestick via sideloading. Depending on the version of Fire OS, it differs which Android applications you can use. You can find the instructions for this on the following page: https://www.lifewire.com/how-to-sideload-fire-tv-apps-on-your-fire-tv-4582671.
After sideloading, it is possible to download GOOSE VPN via the following link: https://portal.goosevpn.com/dl.php?type=d&id=5. If it is not possible to install the GOOSE VPN application, you can use the OpenVPN Connect app manual (https://portal.goosevpn.com/downloads/manuals/GOOSE%20VPN%20OpenVPN%20Android%20%)5BNL% 5D.pdf).