\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary {arrows.meta,graphs,graphdrawing} \usegdlibrary {layered, trees}
\tikzset{nodes={text height=.7em, text width=2.8cm, align=center,
draw=black!50, thick, font=\footnotesize, fill=white},
>={Stealth[round,sep]}, rounded corners, semithick}

\begin{document}


\begin{tikzpicture}
    \draw[fill=lightgray] (-1.7,0.5) rectangle (5, -2.8);
    \draw[fill=lightgray] (-1.7,-3) rectangle (5, -5.5);
    \draw (3.4,0) node[draw=none, fill=none] (wf) {Waterfall};
    \graph [layered layout, level distance=1.2cm, sibling sep=.5em, sibling distance=3.4cm, component order=by first specified node] {
        pd [as=Problem Description,grow=down] -> "Specifications" -> "Initial Design" -> "Feature Separation" -> {Rapid Development ->[bend right] Variable Approach};
        {Rapid Development <-[bend left] Variable Approach};
        {Rapid Interation Design Method[draw=none, fill=none] ->[draw=none] Variable Approach};
        { [same layer] Feature Separation, Rapid Interation Design Method};
        
        %-> tree[fill=none, minimum height=3cm, minimum width=6cm] // [tree layout] {Rapid Development -> Variable Approach};
        { [same layer] Variable Approach, Rapid Development };
    };
\end{tikzpicture}
\end{document}