The observations of M87* probe the spacetime geometry in the region around the photon sphere i.e. around a few Schwarzschild radii. A gravastar has the same spacetime geometry in this region, so the EHT observations cannot distinguish between a gravastar and a Kerr or Schwarzschild geometry.
If we want to dive a little deeper into this things start getting complicated. From the perspective of an external observer the event horizon takes an infinite time to form so our perspective M87* is not a fully grown black hole. It is on its way to forming one, but that process will take an infinite time to complete.
So the spacetime geometry is actually something like a rotating Oppenheimer-Snyder metric (though I don't think any analytic solution is known for this geometry) and it is neither a gravastar nor a Kerr geometry. The only way you could resolve the question would be to jump into M87*, but aside from terminating your academic career this also has the problem that you would be unable to tell the rest of us what you found.