The goal is not just to change tools for the sake of changing tools, as there is a relative consensus that the current tools we have (BZ and Gerrit) are pleasant enough to work with.
The goal is to facilitate contributions and get more contributors, this is one of the key factors the sustainability of any OSS project. So it's not really about GitLab vs GitHub; it's more about exposing code and contribution process on
whatever.eclipse.org vs GitHub. GitHub is the platform where most of OSS projects and contributors are, and that's not a temporary thing now, it's been so for many years and has only grown in size and quality. It's global, bigger than
eclipse.org has ever and will ever be, and it's now the main driver of OSS development practices (contributor agreement and license check bots are basically providing a good enough plug-and-play governance). IMO, the real question is not really about whether to move some process to GitHub, as it will sooner or later be a requirement for project sustainability; it's more about how and when to move code and processes GitHub.
