Download and install latest stable steam client
Let steam bootstrapper update client and log in to your account with remember my password
checked and exit steam client after it will log into your account
Copy steamcclliient.exe into steam install folder root and run it
steamcclliient will register itself as default steam://
protocol handler and will be ready to be used with thirdparty launchers like GOG Galaxy 2.0 ...
steamcclliient <command> <appid>
Available commands are: install
uninstall
and run
( launch
)