Confirmed on amd64 and x86. check attached build log
Created attachment 216121 [details] Build log with failing tests
Created attachment 216123 [details] emerge --info
It does fail tests. Don't really want to fix them, and I'm not sure they are legitimate tests. I've reported this upstream more than a year ago but is fairly quiet.
The test phase been removed from dev-python/pygsl-0.9.5-r1. This bug should be marked WONTFIX and this package should be purged from Portage in lieu of the revised version.
Conclusion courtesy of dugz