\documentclass[12pt,a4paper]{article}
\usepackage[margin=1cm]{geometry}
\usepackage{tikz,tcolorbox}
\usepackage{enumitem}
\usepackage{eso-pic}
\usetikzlibrary{backgrounds}

% Draw background grid
\newcommand{\BackgroundGrid}{%
  \begin{tikzpicture}[remember picture, overlay]
    \begin{scope}[on background layer]
      % small subgrid (optional)
      \draw[step=0.1cm, gray!15, very thin]
        (current page.south west) grid (current page.north east);
      % main grid every 1 cm
      \draw[step=1cm, gray!40, thin]
        (current page.south west) grid (current page.north east);
    \end{scope}
  \end{tikzpicture}%
}

\AddToShipoutPictureBG*{%
  \BackgroundGrid
}


% define tight enemerate on first level only {1}
\newlist{tightenum}{enumerate}{1}
\setlist[tightenum]{label=\textbf{\arabic*)}, leftmargin=*, itemsep=0pt, topsep=0pt, parsep=0pt, partopsep=0pt}

\begin{document}
% Set text line spacing to exactly 1 cm
\setlength{\baselineskip}{1cm}
\vspace*{0.1pt}
This text line sits exactly on a grid line.\\
So does this one.\\
Each line corresponds perfectly to one 1cm step of the background grid.
You can check this visually by printing or zooming in closely.
\begin{tightenum}
    \item this is an enumerate environement redefined
    \item with no vertical skips
\end{tightenum}
in order for this grid template to work avoid items that set their own vertical skips
or redefine them
\begin{tcolorbox}
in order for this grid template to work avoid items that set their own vertical skips
or redefine them
\end{tcolorbox}
in order for this grid template to work avoid items that set their own vertical skips
or redefine them
\textbf{\textcolor{red!50!black}{it is not worth it if you ask me!}}\\
\vspace*{-1mm}

\noindent Or keep correcting it using \verb|\vspace*{-1mm}|



\end{document}