What is Gatsby?
Why learn it?
Being that it's a static site generator, Gatsby is 2 to 3 times faster than most other website stacks. It also comes out of the box fully configured but at the same time it allows for easy customization.
Is it hard?
While it's certainly difficult to learn React, GraphQL and webpack on your own, Gatsby does not make this a requirement. Essentially, you can learn all these new technologies as you learn your way around the Gatsby ecosystem. I will say that while this is certainly possible, as well as it being a selling point for Gatsby, it is recommended that you learn react first.