If you keep updating that project, In that case you can add build event to GAC automatically on that particular project using following post built event.Under Build Event in project properties just you can add the following post build event to GAC your modified dll.Just copy and past following line with zero modification
"%ProgramFiles(x86)%\Microsoft SDKs\Windows\v7.0A\bin\NETFX 4.0 Tools\x64\gacutil.exe" /i "$(TargetPath)"
But make sure that gacutil.exe
in that path.