\documentclass[a4paper,12pt,brazil]{article}

\usepackage[brazil]{babel}
\pagestyle{empty}
\usepackage{gastex}

\usepackage[usenames]{color}

\newcounter{conta} 
 
\begin{document}

        \begin{center}
         \mbox{\setlength{\unitlength}{1mm}
          \begin{picture}(160,46)\setlength{\unitlength}{1mm}
    \put(10,0){\framebox(40,36)}
    \put(25,32){\makebox(0,0){$A$}}
    \put(35,32){\makebox(0,0){$B$}}
    \drawcircle[fillcolor=Gray](25,20,20)
    \drawcircle[fillcolor=Gray](35,20,20)
    \drawcircle(25,20,20)
    \put(30,4){\makebox(0,0){$A\cup B$}}
          \end{picture}
         }
        \end{center}

\end{document}
