My worry is that by making it very clear that Gren is a fork of Elm, people will assume that what works in Elm also works in Gren. It won't.

Gren isn't compatible, and will never be compatible, with Elm. There is different syntax, API's and runtime characteristics. And it will continue to diverge more and more.

That Gren is a fork, was mostly to save development time. You can expect that much of the compiler and core packages to be rewritten in a few years.