documentclass[tikz,margin=5pt]{standalone}
usetikzlibrary{fadings}
tikzfading[name=fade right,
left color=transparent!0,
right color=transparent!100]
begin{document}
begin{tikzpicture}
fill [red,path fading=fade right] (-1,-1) rectangle (1,1);
end{tikzpicture}
end{document}