I\'d like to define an alias that runs the following two commands consecutively.
gnome-screensaver gnome-screensaver-command --lock
Right
So use a semi-colon:
alias lock='gnome-screensaver; gnome-screen-saver-command --lock'
This doesn't work well if you want to supply arguments to the first command. Alternatively, create a trivial script in your $HOME/bin directory.