No, it is not possible.
Github pages is a service to host static pages only. Since nodebb need a database and server side code to run, you can not host it on github.io.
There are some free hosting solutions that you cn use instead like for example openshift (there are more but i dont remember names right now, theres some posts on this community about them, try to search nodebb free hosting).
You can also try cloud9, theres a nice tutorial made by @pichalite Setting up NodeBB on cloud 9 https://community.nodebb.org/topic/7983/setting-up-nodebb-on-cloud-9