are you just attempting a low effort gpl3 FUD or are you honestly confused at something concrete that you failed to describe?
Either way, the GPL 3 was designed with signing already being a thing, GPG had been out for 5+ years. And if signing broke the GPL, I imagine Stallman would have been raising a huge fit about it for decades.
or a closed source including a gpl3 Library in its packaging.
both have nothing with the license and are a common FUD argument by people who do not like the implied freedoms of the gpl when arguing for more permissively comercial licenses, so apologize if i wrongly mistook you for the more probable case.