Gitlab and GitHub tackle same issues and have the same market. As someone said, if you are not violating copyright or patent law, you're good. Ideas are worthless, it's the execution part that matters... If you are better at it you'll win. If the market is big enough for both of you, you won't have problems. Imagine if McDonalds didn't start when they saw KFC was already into fast food business? Or burger king, or if we only had one pizza business. Competition pushes companies forward, having a monopoly is only good for the business, not the users.