\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary {arrows.meta,graphs,graphdrawing,positioning} \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}[on grid,y=1.2cm,x=3.2cm]
    \draw[fill=lightgray] (-1.7cm , 0.5cm) rectangle (5cm, -2.8cm);
    \draw[fill=lightgray] (-1.7cm,-3cm) rectangle (5cm, -7.2cm);
    \input{design_flow.tikz}
    \draw (3.4cm,0cm) node[draw=none, fill=none,text width=2.9cm] (wf) {Systems Engineering:\\Waterfall};
    \node (ridm)[right=1 of fs, fill=none, draw=none] {Rapid iterative Design Method};
\end{tikzpicture}
\end{document}