Please forgive me if i nag.
Here is an API to bind gtk3 to ocaml.
https://garrigue.github.io/lablgtk/lablgtk3/lablgtk3/index.html
Just I have totally no idea how to use it.
Meaning i will go for another language for this kind of problem.
This is the same thing for python,
https://python-gtk-3-tutorial.readthedocs.io/en/latest/install.html
I have to agree, the lablgtk documentation makes it one of the hardest packages in the OCaml ecosystem to pick up (aside maybe from elpi 
).
It would be nice to have an actual example based guide to using Lablgtk, but lacking that, I was able to work out how to use it by referring to the examples on the source repository: lablgtk/examples at lablgtk3 · garrigue/lablgtk · GitHub
This is for LablGtk2, but a lot is usable with v3 as well. May help.
This is for racket (scheme), look at the good documentation.
1 Windowing..Creating.Windows
I am not going to claim that the ocaml documentation is perfect or even above average - although the documentation for the standard library is in my view pretty good - but I don’t think your last one is an entirely fair point.
Racket has its own GUI library which to be usable has to be documented, whereas lablgtk3 is a thin wrapper over the GTK library. GTK is not a library I especially enjoy using, but If you need to know how to write a GTK program the best first resource is the documentation for GTK itself. It is easy enough to work out how that maps onto lablgtk.
For simple cases I have used js_of_ocaml with the gjs/gobject-introspection bindings for GTK3/4 which are also documented, so there are many options. (The python wrapper to which you have referred also uses gobject-introspection.)
[I want to stay away of javascript as it is ugly]
I like the ocaml type system and the gtk3 framework.
All lablgtk examples compiled without problem, including interesting tree-widgets.
Looking at the code of the examples it is somewhat self-documenting.
The code is terse and short which is a good thing.