Although I have gmp installed, I get:
<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
[ERROR] The compilation of conf-gmp failed at "/home/dream/.opam/opam-init/hooks/sandbox.sh build sh -exc cc -c $CFLAGS -I/usr/local/include test.c".
#=== ERROR while compiling conf-gmp.3 =========================================#
# context 2.0.8 | linux/x86_64 | ocaml-system.4.12.0 | https://opam.ocaml.org#b7eb40e7
# path ~/.opam/default/.opam-switch/build/conf-gmp.3
# command ~/.opam/opam-init/hooks/sandbox.sh build sh -exc cc -c $CFLAGS -I/usr/local/include test.c
# exit-code 1
# env-file ~/.opam/log/conf-gmp-75590-e1320e.env
# output-file ~/.opam/log/conf-gmp-75590-e1320e.out
### output ###
# + cc -c -I/usr/local/include test.c
# test.c:1:10: fatal error: gmp.h: No such file or directory
# 1 | #include <gmp.h>
# | ^~~~~~~
# compilation terminated.
<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
┌─ The following actions failed
│ λ build conf-gmp 3
└─
╶─ No changes have been performed