I am pretty sure this is one of the annoyances of GPL. pHash authors explictly licensed it under GPL, and not LGPL because they want any distributable derivative work to be licensed under the same license.
As far as I am not a big fan on GPL for libraries (it's a nasty choice), licensing info of this gem is not proper, and can possibly cause some people trouble.
As bad as it is, it does not mean you can not use the gem and pHash library in closed-source projects and projects you develop for you clients that never get released/distributed outside of that area. GPL does not enforce you to open your modifications to people who only interact with the library. If pHash authors wanted to ensure that, they would have chosen AGPL instead (but man, that would be nasty shit). I would also think about adding such paragraph to README because some people might be confused/repelled if they see GPL licensing in gem.
However, once you start distributing / licensing program which links to GPL licensed library, you are required to release it's sources under GPL to licensees (and no one else).
Changed licensing info. Underlying C library is licensed under GPLv3
(note: not LGPLv3), and this program is linking to it. This means it has
to be licensed under the same license as C library.
Thank you Hubert.