As many have noticed, GSLL does not load with GSL 2.x, because of changes to the library API. I have pushed a commit 32bbd1aae8ef that should address this, and I'd like to find testers who have GSL 2 installed. If you do, and you're comfortable with Lisp programming and loading GSLL from git, please get this commit and try loading it. You should get an error "GSLL does not currently work with GSL version 2.x". Then go into init/gsl-version.lisp and remove the last form (error ...), and try loading again. Let me know what happens.
Note that at this point, just getting GSLL to load is my goal. It may not execute the changed definitions without error, but I want to start by just getting it to load. If you have results on what happens when you run the functions, I'd be happy to hear about it.
Here is the gitlab page for GSLL:
https://gitlab.common-lisp.net/antik/gsll