GitHub command-line client Formula was removed
Last versions were
gh requires the following formula to be installed:
go 1.10 Open source programming language to build simple/reliable/efficient software

Recent formula history

ilovezfs gh: delete (#16903)
Dominyk Tiller gh: don't lean on HOMEBREW_REPOSITORY
Tomasz Pajor gh: audit fixes
Nikolaus Wittenstein Add descriptions to all remaining homebrew packages
Mike McQuaid gh: mark bottle as relocatable.

Formula code at GitHub