You can probably ask the original author

I think Blaze posted some blog post on how to deploy Onion-Services with Torspray a few days ago. And there’s the official TOR documentation. Basically you’d create a hidden service and point it at the port your software opens, or at your reverse proxy. In case of GoToSocial and just mirroring it, I guess it might work pretty much like that. They have some additional info on split-domain deployment in case it needs additional steps or you want a different setup. But I have almost no experience with GoToSocial, take this with a grain of salt.