documentclass{article}
usepackage{everypage}
usepackage{xcolor}
usepackage{lipsum}
% THESE ARE LaTeX DEFAULTS; CAN CHANGE IF NEEDED.
defPageTopMargin{1in}
defPageLeftMargin{1in}
newcommandatxy[3]{%
 AddThispageHook{smash{hspace*{dimexpr-PageLeftMargin-hoffset+#1relax}%
  raisebox{dimexprPageTopMargin+voffset-#2relax}{textcolor{red}{#3}}}}}
% VERIFIED THAT SETTING hoffset AND voffset DO NOT BREAK SOLUTION.
%hoffset=0.4in
%voffset=0.2in
usepackage{tcolorbox}
tcbuselibrary{skins}

newtcolorbox{mybox}[1][6in]{
    enhanced, colframe=none, arc=0pt, width=8.5in,
    top=5.5in,bottom=5.5in,
    #1
}
atxy{0in}{11in}{%
 begin{mybox}[interior style={left color=red!35,right color=red!5}]end{mybox}%
}
begin{document}

lipsum[1]

end{document}