Sometimes, best answers come after re-reading the text of the license itself, more carefully. Now, I have to admit that I might be biased towards scenario 2( possible).
For the sake of accuracy, I am talking about text of GPL 2.0, used by https://github.com/metaregistrar/php-epp-client/blob/master/LICENSE.md
The key clause is the following: “Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.”
The essence of GPL it’s ABOUT DISTRIBUTION, NOT RUNNING the program
The optimistic scenario would be:
-Can’t distribute WHMCS PROPRIETARY program + GPL WHMCS module using GPL library.
but
-Can distribute only GPL WHMCS module using GPL library, and can run WHMCS PROPRIETARY program + GPL WHMCS module using GPL library, because running the combination is outside the scope of GPL
Worst case scenario, only WHMCS itself, as a company, couldn't distribute GPL WHMCS module using GPL library, because they charge for the proprietary license. But me, as a third party that doesn't sell WHMCS licenses, I am ok to distribute only the module with gpl library.
1) To be totally paranoid, the WHMCS GPL Module including the GPL library should not be distributed on https://marketplace.whmcs.com/, github being a safer option, like these guys do https://github.com/search?utf8=✓&q=whmcs+license%3Agpl&type=
Or it would be ok to link to github, from whmcs marketplace, in order to have a better visibility at https://marketplace.whmcs.com/category/top-free ?
2) If you break down the clause above, there’s still an exception regarding the program output. https://www.gnu.org/licenses/gpl-faq.en.html#GPLOutput and https://www.gnu.org/licenses/gpl-faq.en.html#WhatCaseIsOutputGPL contain answers.
This isn’t our use case… Here’s a relevant example: “As the authors state, copyright does not provide a means to restrict the output of a program to the licence terms that apply to the program itself. That is only possible if the output consists of part of the source code of the GPL'd program. (For instance, YACC or Bison generate source code for lexers and parsers, and their output contains large parts of the source code of YACC and Bison themselves.)” https://softwareengineering.stackexchange.com/questions/311084/using-output-of-gnu-gpl-software-in-commercial-purposes
3) I guess that no announcement is required to print “if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.”
4) That are still some questions:
-What about the incorporation clause? “This General Public License does not permit incorporating your program into proprietary programs.” On the other hand we have “Activities other than copying, distribution and modification are not covered by this License; they are outside its scope.” Which clause takes precedence? If the last one, it means that we can’t incorporate and distribute, but when it comes to running on own server, we can mix it !
Confirmation from http://blog.milkingthegnu.org/2008/04/gpl-for-dummies.html “This DOES mean that I can combine any proprietary software I have a license for with any GPL software I wish and use it as I please as long as I don't redistribute any of it.”
5) “If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works.” We can’t invoke this clause, but there’s no need to do it, because we don’t distribute the proprietary system.
6) Finally, https://github.com/Frikkadel/whmcs-zacrcoza is already using “Metaregistrar's Ewout de Graaf's excellent php-epp-client library.” This fact doesn’t make it more “legal”, but it’s a precedence.