Written in 2022, 4 years ago. Tools and advice may have moved on since.
As mentioned in Stripe Event validation in a Gatsby Serverless Function, getting the raw body in a Gatsby Serverless Functions did not work in development.
Luckily the framework team at Gatsby agreed it was a bug and fixed it. It looks like it will ship with Gatsby v4.16.
I got it working today using 4.16.0-next.1.
yarn add gatsby@4.16.0-next.1
All the best,
Queen Raae