What i mean is am i free to use other free services like github pages to host my projects?
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As long as the work and code are viewable, you can use whatever you like.
Lots of us use gh-pages 
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If you can, use github pages.
Codepen is suggested so that new coders can jump right into projects without worrying too much about configuration.
Use anything you like. I do all of my FCC projects locally and host them on my own website. When I submit a finished project, I just put in my actual web URL instead of a link to codepen. No problems for me so far!
The project links are evaluated. We click the link review the project. I also suggest when not using codepen or other simple front end technologies (back end stuff) putting a github repo link in the project under about. Personally, I will be working on the Rogue-like dungeon crawler on an express server for hosting micro-API’s and hosting on Heroku. So it is personal preference.
From experience, when I check projects for green lighting for non-profit project source code is reviewed, I generally look for github links to verify that the code it all yours, or attributed.
This is in the Frequently Asked Questions: About freeCodeCamp - Frequently Asked Questions
#Do I have to use CodePen for the front end projects?
As long as your code is publicly viewable, and you have a live demo, you can use whatever you want.