Is it possible to define a command in bash?

just type:

alias gb='cd /media/Dan/evolution'

To make this setting permanent (so that it sticks after you restart or open another console) add this line to the file ~/.bashrc (assuming you use the bash as your default shell)

Alternative to aliasing

gb() { cd /media/Dan/evolution; }

This defines shell function gb, which takes no arguments, and performs cd /media/Dan/evolution. As with other suggeststions, this can be added to ~/.bashrc

It is possible, and alias is the command you're looking for. For example alias ll="ls -l" in bash will let you type ll instead of ls -l. Please note there are no spaces used when setting an alias. man alias can be useful as well.