Hi Christophe,
I have found the solution, that is by adding this (below) in the _output.yml
pandoc_args: --top-level-division=chapter
So, the _output.yml becomes
bookdown::gitbook:
config:
toc:
download: [pdf, epub]
bookdown::html_chapters:
css: [style.css, toc.css]
bookdown::pdf_book:
includes:
in_header: preamble.tex
before_body: before_body.tex
after_body: after_body.tex
latex_engine: xelatex
citation_package: natbib
keep_tex: yes
pandoc_args: --top-level-division=chapter
toc_depth: 3
toc_unnumbered: no
toc_appendix: yes
bookdown::epub_book:
dev: svglite
stylesheet: style.css