How to use gray colour mode in fill command in Tikz

This seems like a really stupid question, but I cannot find the answer…

I am trying to make a contiguous grid of coloured blocks using Tikz, much like in this question, but in my case, the colours to be filled in are grayscale values in the real number range [0.0,1.0], i.e. the ‘gray’ colour model listed in the xcolor package documentation. The values (and, in theory, the figures) are generated programmatically and could be any arbitrary number between 0 and 1 (inclusive), so it won’t make sense to use a handful of definecolor commands at the start. Instead, my current plan is to overwrite the fill for each node as my script writes it out to the file.

For a minimum working example, the below produces something that looks like what I want (see the image below it), except I cannot work out how to get the correct sort of grayscale settings. Using the approach I thought might work, the first three cells just produce white (I suspect it is using the Gray system, with an integer range of [0,255]). The latter six create the sort of results I’m looking for, but the fill colours are not specified in a useful way.

documentclass[tikz]{standalone}
usetikzlibrary{positioning}

begin{document}
begin{tikzpicture}[outer sep=0pt,minimum size=1cm]
selectcolormodel{gray}
    node [fill={gray!0.1}] at (0,0) {};
    node [fill={gray!0.2}] at (0,-1) {};
    node [fill={gray!0.3}] at (0,-2) {};
    node [fill={rgb:black,4;white,8}] at (1,0) {};
    node [fill={rgb:black,5;white,7}] at (1,-1) {};
    node [fill={rgb:black,6;white,6}] at (1,-2) {};
    node [fill={rgb:black,7;white,5}] at (2,0) {};
    node [fill={rgb:black,8;white,4}] at (2,-1) {};
    node [fill={rgb:black,9;white,3}] at (2,-2) {};
end{tikzpicture}
end{document}

I have also tried out such commands as gray:0.1, or using PassOptionsToPackage{gray}{xcolor} and then something like just 0.1, but all of those seem to hit compilation errors.

Alternatively, I also tried using the ‘matrix’ option of Tikz, as below, but that didn’t seem to permit any colouring of the grid cells (the visible lines come from using the node=draw option).

documentclass[tikz]{standalone}
usetikzlibrary{positioning}

begin{document}
begin{tikzpicture}[nodes=draw]
    matrix {
        node { }; [fill={rgb:black,2;white,10}]; & node {}; [fill={rgb:black,3;white,9}]; & node {}; [fill={rgb:black,4;white,8}]; 
        node { }; [fill={rgb:black,5;white,7}]; & node {}; [fill={rgb:black,6;white,6}]; & node {}; [fill={rgb:black,7;white,5}]; 
        node { }; [fill={rgb:black,2;white,10}]; & node {}; [fill={rgb:black,3;white,9}]; & node {}; [fill={rgb:black,4;white,8}]; 
    };
end{tikzpicture}
end{document}

How can I create a grid of grayscale coloured blocks using Tikz, with gray colour values in the real number range [0.0,1.0] specified individually for each cell?

documentclass[tikz]{standalone}
usetikzlibrary{matrix}

begin{document}
begin{tikzpicture}[g/.style={minimum size=1cm,fg=#1},
    fg/.code={pgfmathtruncatemacro{iFill}{100*#1}%
        tikzset{fill=black!iFill}}]
    matrix[matrix of nodes,column sep=0pt,row sep=0pt,nodes in empty cells] {
     |[g=0.3]| & |[g=0.6]|
     |[g=0.1]| & |[g=0.9]|
     |[g=0.5]| & |[g=0.2]|
    };
end{tikzpicture}
end{document}

documentclass[tikz,border=2mm]{standalone}
usetikzlibrary{matrix}
begin{document}
begin{tikzpicture}[
    mycolor/.code={
        pgfmathsetmacro{percent}{(pgfmatrixcurrentrow-1)*20+(pgfmatrixcurrentcolumn-1)*5},
        tikzset{fill=gray!percent},
        }
    ]
matrix[matrix of nodes, column sep=0pt, row sep=0pt,
    nodes in empty cells,
    nodes={minimum size=1cm, outer sep=0pt, mycolor}
    ]{
    & & & 
     & & & 
     & & & 
     & & & 
    };
end{tikzpicture}
end{document}