I honestly didn't even know that was a thing.
That said, it looks like it doesn't support JSONPath out of the box, though I was able to find a plugin that should allow you to parse JSON. I'm not sure how that would work out, but the point is that this is all-in-one.
Beyond just that, the resource usage of Gatus is significantly lower than running Grafana, Prometheus and alertmanager individually - much less running all of them at the same time.
My personal Kubernetes cluster is quite limited in resources after all, and running Prometheus does make that resource pool even smaller than it already is :p
That said, even for clusters that don't have similar resource restrictions, I find that having all of this together in a single application makes it a lot easier to manage, though I guess that's up to the individual :)