The gpgsign feature is not available via bashcompletion on git.

$ git config --global commit.
commit.status     commit.template   
$ git config --global commit.gpgsign true
$ git --version
git version 2.10.2

Due to this I could also find no option to automatically sign every tag
that I do. If this option does not exist yet, please add it to the todo
list. If those reports are tracked anywhere else, please also let me
know, so I can test that out.

Cheers,
Nico