the date binary of Os X is not fully compliant with gnu date, and build fails.
as quick fix, one can install gdate with brew install coreutils, and patch the build.bash formula with brew edit.

here is my full formula for those willing to try the 2.0b4

class Gocryptfs < Formula
desc "Encrypted overlay filesystem written in Go"
homepage "https://nuetzlich.net/gocryptfs/"
url "https://github.com/rfjakob/gocryptfs/releases/download/v2.0-beta4/gocryptfs_v2.0-beta4_src-deps.tar.gz"
sha256 "d28c6b6a749e9f66745bd9a595b853b1ffde7ca3b2e82e0a41e9b3c40e7ebddd"
license "MIT"

depends_on "go" => :build
depends_on "pkg-config" => :build
depends_on "openssl@1.1"

on_macos do
end

on_linux do
depends_on "libfuse"
end

def install
inreplace "./build.bash",
"date ",
"/usr/local/bin/gdate "

ENV["GOPATH"] = buildpath
(buildpath/"src/github.com/rfjakob/gocryptfs").install buildpath.children
cd "src/github.com/rfjakob/gocryptfs" do
  system "./build.bash"
  bin.install "gocryptfs"
  prefix.install_metafiles
end

end

test do
(testpath/"encdir").mkpath
pipe_output("#{bin}/gocryptfs -init #{testpath}/encdir", "password", 0)
assert_predicate testpath/"encdir/gocryptfs.conf", :exist?
end
end