Of course rendering direct to avi (without need for using the blender video editor) and use mplayer or alike to produce the gif is also a possibility.
This tutorial is for people, who are just through the Mancandy level. My intention was to keep things easy. Using Gimp is easy - in my opinion and you have full control about every frames display length. I should have mentioned it - you can set the display time for every frame individual in the layers dialog.
Another reason NOT to render directly into avi is that single images are quite more handy. You can check images for mistakes, you have more freedom in post processing, and so on. I never use direct avi rendering.
The time needed for opening the images in gimp and saving them as gif is not so much more than importing an avi in a video editor and render them to gif. Most video editors even don’t support gif - kdenlive, kino, lives, pitivi. The result of the mpeg conversion looks grainy.
Once again: with gimp things stay simple, results are good, and you have control about every frame.