You can draw squares with shading inside or you can draw functions and use pgfplots
heatmaps to get the shading. This is probably total overkill and way slower than it should be:
\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{
compat=newest,
heatmap/.style={
view={0}{90},
scale only axis=true,
domain=-1:1,
domain y=-1:1,
width=3cm,
height=3cm,
xtick={0},
xticklabels={},
ytick={0},
yticklabels={},
yticklabel style={overlay},
grid=major,
grid style={thick,purple},
axis line style={purple},
colormap/blackwhite,
},
}
\begin{document}
\begin{equation}
\begin{tikzpicture}[baseline=(C.base)]
\begin{axis}[heatmap,xlabel={$f(x)$}]
\node (C) at (axis cs:0,0) {\strut};
\addplot3 [surf,shader=interp] {exp(-x^2)};
\end{axis}
\end{tikzpicture}
\times
\begin{tikzpicture}[baseline=(C.base)]
\begin{axis}[heatmap,xlabel={$f(y)$}]
\node (C) at (axis cs:0,0) {\strut};
\addplot3 [surf,shader=interp] {exp(-y^2)};
\end{axis}
\end{tikzpicture}
=
\begin{tikzpicture}[baseline=(C.base)]
\begin{axis}[heatmap,xlabel={$f(x) f(y)$}]
\node (C) at (axis cs:0,0) {\strut};
\addplot3 [surf,shader=interp] {exp(-x^2-y^2)};
\end{axis}
\end{tikzpicture}
\end{equation}
\end{document}

shading
TikZ library is your friend.