Commands are not distinguished from options except for the fact that only one command is allowed.
stdin. The default mode is to create a socket and listen for commands there.
--daemon [command line
eval $(gpg-agent --daemon)to setup the environment variables. The option --write-env-file is another way commonly used to do this. Yet another way is creating a new process as a child of gpg-agent:
gpg-agent --daemon /bin/sh. This way you get a new shell with the environment setup properly; if you exit from this shell, gpg-agent terminates as well.