Below the graph on the main screen of Pronterface, you will see a little (+) button or maybe a (-) if you're on mac.
Click the button and type "Fan On" for the button title. For the command, type "
M106 S255" and for the color pick green.
You may have to resize your window to see your new button. Let's do the same for the "Fan Off" button. Click the button again and this time name is "Fan Off." The command will be "M107" and lets pick Red for the color.
That's it! You now have custom fan buttons that you can use while your Gigabot is connected to Pronterface.