Thank you for the replies and suggestions.

I am fairly new to this, and just getting the idea of how to track these problems down and fix them.
After finding, and digging through the git pages and code I believe up to version 2.6 GSL is supported.

https://github.com/leto/math--gsl/blob/master/developer/wiki/Upgrade.md
https://github.com/leto/math--gsl/blob/master/inc/Ver2Func.pm

I have downloaded a 2.6 GSL version from the GNU ftp site supporting GSL, and have been working on compiling/installing etc.
Commenting out the version check is a great idea I hadn't thought of. May not use it, but will keep the idea for future debugging problems.
Assuming the 2.6 build goes okay, all I am left with figuring out is how to specify using the previous 2.6 GSL version for the build, and my scripts.