My time and companies time is better spent working on our core product. Can I setup a VPS server with Gogs? Of course, but that is wasted bandwidth and energy. Less than $2 a day for GitHub a tool that I use daily and religiously is great value.
Curious, have you tried running your own startup before?
The most value in GitLab today is that you have many things that requires you to setup additional integration with GitHub, as part of the product, and because of that you can get complex workflows mapped easily.
For example, I personally opt for self hosting in many cases where I need more control over what runs on the server. It saves me a lot of time if I can do this; cost savings is a side benefit.