\documentclass[12pt]{article}

\def\bmstyle#1#2{\mbox{\boldmath$#1#2$}}
\def\bm#1{\mathpalette\bmstyle{#1}}

\newcommand{\btX}{ {\bm X} }
\newcommand{\btW}{ {\bm W} }
\newcommand{\btY}{ {\bm Y} }
\newcommand{\btZ}{ {\bm Z} }
\newcommand{\bG}{\mathbf{G}}

\begin{document}
\newcommand{\zap}{~~~~~~~~~~~~~~~~~~~~}

\[
\begin{array}{ccc}
\bG^{(Y,o)}_{t,2}	&	\bG^{(Y,o)}_{t,3}	&	\bG^{(Y,o)}_{t,4}	\\
\\
\\
\bG^{(Y,\ell)}_{t,2}	&	\bG^{(Y,\ell)}_{t,3}	&	\bG^{(Y,\ell)}_{t,4}	\\
\\
\\
\btX_{t,2}	&		&			\\
	&	\btX_{t,3}	&		\\
	&		&	\btX_{t,4}		\\
\\
\\
\\
	&	\btW_{t}	&			\\
\\
\\
	&	\bG^{(Z,\ell)}_{t}	&			\\
\\
\\
	&	\bG^{(Z,o)}_{t}	&			\\
\end{array}
\]

\[
\begin{array}{ccc}
\btY_{t,2}	&	\btY_{t,3}	&	\btY_{t,4}	\\
\\
\\
\btX_{t,2}	&		&			\\
	&	\btX_{t,3}	&		\\
	&		&	\btX_{t,4}		\\
\\
\\
\\
	&	\btW_{t}	&			\\
\\
\\
	&	\btZ_{t}	&			\\
\end{array}
\]

\end{document}


