documentclass[10pt]{article}
usepackage[margin=2cm]{geometry} % just for the example
usepackage[frenchb]{babel}
usepackage[table]{xcolor}
usepackage{array}
usepackage{tikz}
usepackage{lipsum}
usetikzlibrary{calc,shadings}

% Andrew Stacey's code from
% http://tex.stackexchange.com/a/50054/3954
makeatletter
tikzset{%
  remember picture with id/.style={%
    remember picture,
    overlay,
    save picture id=#1,
  },
  save picture id/.code={%
    edefpgf@temp{#1}%
    immediatewritepgfutil@auxout{%
      noexpandsavepointas{pgf@temp}{pgfpictureid}}%
  },
  if picture id/.code args={#1#2#3}{%
    @ifundefined{save@pt@#1}{%
      pgfkeysalso{#3}%
    }{
      pgfkeysalso{#2}%
    }
  }
}

defsavepointas#1#2{%
  expandaftergdefcsname save@pt@#1endcsname{#2}%
}

deftmk@labeldef#1,#2@nil{%
  deftmk@label{#1}%
  deftmk@def{#2}%
}

tikzdeclarecoordinatesystem{pic}{%
  pgfutil@in@,{#1}%
  ifpgfutil@in@%
    tmk@labeldef#1@nil
  else
    tmk@labeldef#1,(0pt,0pt)@nil
  fi
  @ifundefined{save@pt@tmk@label}{%
    tikz@scan@one@pointpgfutil@firstofonetmk@def
  }{%
  pgfsys@getposition{csname save@pt@tmk@labelendcsname}save@orig@pic%
  pgfsys@getposition{pgfpictureid}save@this@pic%
  pgf@process{pgfpointoriginsave@this@pic}%
  pgf@xa=pgf@x
  pgf@ya=pgf@y
  pgf@process{pgfpointoriginsave@orig@pic}%
  advancepgf@x by -pgf@xa
  advancepgf@y by -pgf@ya
  }%
}
newcommandtikzmark[2][]{%
tikz[remember picture with id=#2] {#1;}}
makeatother
% end of Andrew's code

newcommandShadeCell[4][0pt]{%
  begin{tikzpicture}[overlay,remember picture]%
    shade[#4] ( $ (pic cs:#2) + (0pt,2ex) $ ) rectangle ( $ (pic cs:#3) + (0pt,-#1*baselineskip-.8ex) $ );
  end{tikzpicture}%
}%

begin{document}

ShadeCell[14]{start1}{end1}{%
  shading=color wheel white center,opacity=.15}
ShadeCell{start2}{end2}{%
  left color=red!20,right color=green!20}
ShadeCell[13]{start3}{end3}{%
  top color=green!20,bottom color=red!20}
ShadeCell{start4}{end4}{%
  left color=blue!20,right color=green!20}

begin{tabular}{| l | p{6cm} | c |}
hline
Uncolored cell
  & multicolumn{1}{!{tikzmark{start1}} p{6cm} !{vruletikzmark{end1}}}{lipsum*[2]}
  & Uncolored cell \
hline
multicolumn{1}{!{vruletikzmark{start2}} l !{vruletikzmark{end2}}}{Another colored cell}
  & Another uncolored cell & Another uncolored cell \
hline
Uncolored cell
  & lipsum[4]
  & multicolumn{1}{!{tikzmark{start3}} c !{vruletikzmark{end3}}}{Another colored cell} \
hline
multicolumn{2}{!{vruletikzmark{start4}} c !{vruletikzmark{end4}}}{Another merged colored cell}
  & Uncolored cell \
hline
end{tabular}

end{document}

!{tikzmark{<name1>}} <format> !{tikzmark{<name2>}}

!{vruletikzmark{<name1>}} <format> !{vruletikzmark{<name2>}}

ShadeCell{<name1>}{<name2>}{<shade specification>}

ShadeCell[4]{<name1>}{<name2>}{<shade specification>}

documentclass{article}
usepackage{tikz}
usetikzlibrary{matrix}

begin{document}

begin{tikzpicture}

matrix (A) [matrix of nodes,
             row 2 column 2/.style={ nodes = { top color=blue!20, bottom color=red!20 }}]
{
A & B & C \
D & E & F \
G & H & I \
};
end{tikzpicture}

end{document}

documentclass{article}
usepackage{tikz}
usetikzlibrary{matrix,calc}

begin{document}

begin{tikzpicture}
matrix (A) [matrix of nodes, nodes in empty cells,
             text height=9pt, text depth = 1pt,
             row 3 column 1/.style={
               nodes = { top color=green!20, bottom color=red!20 }},
             row 2 column 2/.style={
               nodes = { left color=blue!20, right color=red!20 }},
             row 1 column 3/.style={
               nodes = { top color=blue!20, bottom color=red!20 }},
             column 1/.style={ text width=15mm, align=left },
             column 2/.style={ text width=15mm, align=right },
             column 3/.style={ minimum width=20mm} % centered!
             ]
{
Lorem & ipsum & dolor \
sit & amet & consectetur \
adipiscing& {}  & {} \
};
% Draw some lines
draw (A-1-1.north east) -- (A-3-1.south east);
draw[thick, dotted] (A-2-2.south west) -- (A-2-3.south east) -- (A-1-3.north east);
% Faked multicolumn
node[text height=9pt,text depth=1pt] at ($(A-3-2)!0.50!(A-3-3)$) { Multicol text here };
end{tikzpicture}

end{document}