brew search date unsuccessfully.
coreutils in my system.
I could not find GNU date anywhere.
How can you have GNU date in OS X?
gdate binary from coreutils should be the gnu date program:
$ gdate --version date (GNU coreutils) 8.25 Copyright (C) 2016 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Written by David MacKenzie.
If you don't have that in the path, perhaps
brew rm coreutils && brew install coreutils would be a place to start as well as inspecting your