\documentclass{standalone}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}

\begin{document}

\begin{tikzpicture}
\begin{axis}[
  axis equal, 
  xtick distance=1,
  ytick distance=1,
  tick label style={font=\scriptsize},
  colormap={mygray}{color=(gray!20) color=(black)},
  colorbar,
  only marks,
  mark size=1.75pt,
  scatter,
  point meta=explicit,
]
\addplot table[x=x, y=y, meta=z]{punkty.txt};
\end{axis}
\end{tikzpicture}

\end{document}