LablGTK "unbound module Gmain"

Hi guys, using Ocaml for a while now, as I have to make a little program for the end of this semester in Ocaml.
This is however the first time I have to deal with an external library :
I installed lablgtk with opam, so the lablgtk2 directory is in /usr/lib/ocaml/ (I’m running on Ubuntu 17 04, Ocaml 4 02).
But I everytime i try to compile a with the command :
ocamlc -I +lablgtk2 -o test lablgtk( . )cma test( . )ml (sry i didn’t know how to “embed” code on this forum)
The error : “Unbound module Gmain” pops up. mostly contains
let () = Gmain( . )init () (again embedding code)
at least this is the source of the error.
I can’t find any answer on the internet even though a few people (mostly in french) already had this problem, it never seems to be corrected…

(Please do not feel offended, i created the same post on r/ocaml a few minutes back)

[Kinda Solved]

Works fine with -I $(opam config var lib)/lablgtk2