You don't have to make it "Open Source" to keep it on GitHub. You can make it a private repo and allow select people access to the source.
However, I have observed in history books how the things that were made easily and readily available to the general people were the ones that history remembers. Those advances which were kept behind greedy hands rarely (if ever) became known.
Take, for example, our knowledge from Roman engineering. We would not have built such great bridges if it were not for the free knowledge of smart engineers of ancient past. Perhaps you are afraid that no one will compensate you for your time? Or, do you fear that everyone will "steal" your work? If everyone sees your great work, it will be undeniable who the credit belongs.