1. DevSite Logo Generator.
This is the official page where general guidelines about GDG Logos are defined. The latest GDG icon can be found here. Apart from the guidelines it also includes a logo generator.
You can also use these design resource files if you feel adventurous.
2. Unofficial Logo Generator
There is also another great open source logo generator project by @abdonrd
There is a DEMO part where you can very easily generate a logo for your chapter. It also gives you a Polymer element that you easily use in your Polymer projects.
The source code and information about the Polymer element usage can be found here: