What's difference between stacked PRs and merge trains in gitlab?

Merge trains are an integration method. In GitHub that’s called merge queues.

Stacked PRs are a development method, for managing changes which are separate but dependent on one another (stacked).

The two are orthogonal they can be used together or independently (or not at all).

	▲	IshKebab 8 hours ago \| parent [-]
		Yes, except with a proper UI. Also while you could do this on GitHub before in the same way that that Gitlab feature works, it didn't work cross-repo so in practice it wasn't an option for most open source code. I can't remember if Gitlab has the same limitations but I do remember trying to use Gitlab's stacked diffs and finding them to not work very well. Can't remember why tbh.