Table coloring doesn't extend all the way with makecell

I would not use makecell to do such things.

\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{array}

\rowcolors{2}{gray!20}{white}
\newcolumntype{C}{>{\centering\let\newline\\\arraybackslash\hspace{0pt}}m{2cm}}
% https://tex.stackexchange.com/a/12712/156344
\begin{document}
\begin{tabular}{|C|C|} 
    \rowcolor{gray!50}
    \hline
    \textbf{One} & \textbf{Two}\\
    \hline
    Text & More Text\\
    Even More & Multiple Lines\\
    \hline
\end{tabular}
\end{document}

You could use \Gape.

\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{makecell}

\rowcolors{2}{gray!20}{white}

\begin{document}
    \begin{tabular}{ |c|c| } 
    \rowcolor{gray!50}
    \hline
    \textbf{One} & \textbf{Two}\\
    \hline
    Text & More Text\\
    Even More & \Gape[0pt][2pt]{\makecell{Multiple \\ Lines}}\\
    \hline
    \end{tabular}
\end{document}