In October 2020 I did a deep analysis on commercially supported and/or hosted Kubernetes distributions and platforms (think Rancher, OpenShift, Tanzu, EKS, GKE, etc.) and after doing most of my analysis, a Gartner report found its way to me. It largely matched my results with some skewing for factors I don't consider as important, but, were clearly explained in the report.

So, while I realize this is a small data point, it is one backed by quite a bit of data/analysis. Their report was well reasoned and accurate given the stated priors.

I would say that their credibility is fine so long as you read the reports fully and not simply glance at their magic quadrant figures.