That would be there: https://trisquel.info/node/add/project-issue/trisquel
It would help to discover whether the bug still exists on Trisquel 9, trying to install libgnutls-dev on the live system: http://jenkins.trisquel.info/makeiso-etiona/iso/
If the bug does not exist anymore, it is probably not worth fixing it (apparently, nobody but you has faced it, in more than two years of existence of Trisquel 8). If it still exists, it would be be good to discover whether it exists as well on Ubuntu 18.04 (Trisquel 9 is based on it), trying to install libgnutls-dev on Ubuntu's live system. If Ubuntu 18.04 suffers from the bug, it should be reported and fixed upstream (and Trisquel 9 would profit from the correction).
