How to write underneath $\bigcup$

Question

I try to write something like:

\[
\bigcup_{\gamma<\omega*2}\mathcal{T}_{\gamma}|_{\aleph _0}
\]

But it can not display $\gamma<\omega*2$ underneath \bigcup in TeXstudio. What is the problem?

Here is the whole file in TeXstudio.

\documentclass[preview,border=3pt,3pt]{standalone}

\usepackage{tikz}
\usetikzlibrary{shapes,backgrounds}
\usetikzlibrary{calc}
\usetikzlibrary{decorations.shapes}
\usetikzlibrary{arrows.meta}
\tikzset{decorate sep/.style 2 args=
    {decorate,decoration={shape backgrounds,shape=circle,shape size=#1,shape sep=#2}}}
\tikzset{>={Latex[width=1.5mm,length=3mm]}}

\begin{document}
    \begin{tikzpicture}
    \tikzstyle{line} = [fill=white, line width=.3pt]
    \tikzstyle{gray1} = [fill=gray!5,fill opacity=0.5]
    \tikzstyle{gray2} = [fill=gray!20,fill opacity=0.5]
    \tikzstyle{blue1} = [fill=blue!10,fill opacity=0.5]
    \tikzstyle{blue2} = [fill=blue!20,fill opacity=0.5]


    \filldraw[gray2](0, 9)--(-1.67,6.5)--(-1.33,4)--(5.33,4)--(7,9);   
    \filldraw[blue1](-3, 9)--(-1.33,4)--(-2,9);   
    \filldraw[blue2](-2, 9)--(-1.67,6.5)--(0,9);   

        \node[above] at (-1.2,8.3) {$\bigcup_{\gamma<\omega*2}\mathcal{T}_{\gamma}|_{\aleph _{0}}$};

    \end{tikzpicture}
\end{document}