Always use ocamlfind as a wrapper for ocaml(c|opt), if you use libraries installed via ocamlfind (nearly all libraries provide ocamlfind support nowadays). Only ocamlfind can parse the 'META'-files of these libraries. And ocamlfind can install libraries to different locations (depending on your findlib.conf and certain environmental varibles), plain ocaml possibly won't find them with the '+lib'-notation.
The following should work:
ocamlfind ocamlopt -package lablgtk2.auto-init -linkpkg base.ml -o myprog.exe
ocamlfind ocamlc -package lablgtk2.auto-init -linkpkg base.ml -o myprog_byte.exe
'.auto-init' will add gtkInit for you.