That formula doesn't link - you can do it by hand if you require it:
This formula is keg-only, which means it was not symlinked into /usr/local, because macOS provides the BSD gettext library & some software gets confused if both are in the library path.
The maintainers recommend:
If you need to have this software first in your PATH run:
echo 'export PATH="/usr/local/opt/gettext/bin:$PATH"' >> ~/.bash_profile
You can even get a more explicit warning by trying to link it manually:
$brew link gettext.
Warning: gettext is keg-only and must be linked with --force.
Note that doing so can interfere with building software.
If you're sure you need it linked everywhere:
brew link gettext --force
See this SU thread for details https://superuser.com/questions/747324/brew-install-gettext-should-i-force-link-it