Hasura's homepage[0] links to their Github[1], so it's definitely open source. The "Enterprise" section of the website seems a bit lite and all the pricing information is on the "Support" tab, so the business model seems to be support-based. I only skimmed so I can't speak to what missing features there might be if you roll-your-own.
[0]: https://hasura.io/
[1]: https://github.com/hasura/graphql-engine