Start Gnome Terminal. Select "Edit", then "Profiles...". If you haven't made other profiles, select "Default", then "Edit".
On the "General" tab, uncheck the checkbox "Use the system fixed width font". Now you can select what font to use for Gnome Terminal.
If you want to change the "system fixed width font" for Gnome system wide, select Preferences from the System menu, then Appearance. Go to the Fonts tab and change "Monospace 10" into something else.
To add fonts, you can make a directory called ~/.fonts and put your font files into it.