1. Download and install .NET 5.0 SDK (preview) https://dotnet.microsoft.com/download/dotnet/5.0
2. Choose OS you want to create single file for (in .csproj element <RuntimeIdentifier>) currently set to Linux
3. Publish in release mode dotnet publish -c Release

Should restore, build and publish the gui executable to bin/Release/net5.0/linux-x64/publish