For what it's worth, GNU APL is GNU because the developer chose to associate their project with GNU, not because there was any sort of effort by GNU as an organization to implement APL (see also [0]). It implements what I think is a pretty outdated APL (e.g. dynamic scoping everywhere, no control structures for loops), and I'd say there are better free options now, like Kap and April, and some others listed at [1].

[0] https://en.wikipedia.org/wiki/List_of_GNU_packages#What_it_m...

[1] https://aplwiki.com/wiki/List_of_open-source_array_languages