$ opam depext --version
0.9.1

$ opam depext conf-gmp -n
# Detecting depexts using flags: x86_64 osx homebrew
# The following system packages are needed:
#  - gmp

$ opam depext conf-gmp.1 -n
# Detecting depexts using flags: x86_64 osx homebrew
# The following system packages are needed:
#  - gmp

$ opam depext conf-gmp=1 -n
# Detecting depexts using flags: x86_64 osx homebrew
# The following system packages are needed:
#  - gmp

$ opam depext 'conf-gmp>=1' -n
# Detecting depexts using flags: x86_64 osx homebrew
# No extra OS packages requirements found.