To be fair, GitPod claims they will support devcontainer.json: https://github.com/gitpod-io/gitpod/issues/7721
Do they? It reads more like a request from the community, but without any statement from Gitpod itself.
Sometimes you need to name something and give it a spec for others to implement it.
As I've blogged about here [1], there are other specs such as Devfile.io or GitPod's proprietary gitpod.yml file.
[0] https://xkcd.com/927/
[1] https://www.augmentedmind.de/2022/10/30/container-based-deve...