Thanks!
I could only find this part regarding libgsl.so in the installation guide (Ubuntu18):
sudo ln -s /usr/lib/x86_64-linux-gnu/libgsl.so.23 /usr/lib/x86_64-linux-gnu/libgsl.so.19
That does not cover linking libgsl.so.19 (or libgsl.so.23) to libgsl.so.0.
The installation guide worked nicely when I installed AFNI and this libgsl.so.0 issue surfaced after I updated AFNI. Prior to the update e.g. skullstrip worked fine.
I already hold this "Paul Taylor" to high standards since 2015 when he made my last minute trip to the boot camp in Cape Town work out superbly. Including sightseeing!