Spanning/centering elements with Grid/GraphicsGrid when there's a non-equal number of elem. in rows

Maybe

Column[Row[#, Spacer[5]] & /@ {{a, b, c}, {d, e}},  Alignment -> Center]

{a, b, c, d, e, f, g, h, i} = 
  Table[With[{data = MapIndexed[Flatten[{##}] &, RandomReal[1, {100, 2}]], 
     color = RandomChoice[ColorData["Gradients"]], 
     intord = RandomInteger[{0, 4}]}, 
    ListDensityPlot[data, InterpolationOrder -> intord, ColorFunction -> color,
     Mesh -> All, ImageSize -> 200, ImagePadding -> {{20, 5}, {20, 5}}, 
     PlotLegends -> BarLegend[Automatic, LegendMarkerSize -> 200]]], {9}];

Column[Row[#, Spacer[5]] & /@ {{a, b, c, d}, {e, f, g}, {h, i}}, Alignment -> Center]

Well, what you want is not really a simple grid, because its irregular, but a combination of two grids:

Grid[{{Grid[{{a, b, c}}]}, {Grid[{{d, e}}]}}]

The other thing you can try is to construct it by rows and columns:

Column[{Row[{a, b, c}, " "], Row[{d, e}, " "]}, Center]

Here's nice overview of all this stuff:

https://reference.wolfram.com/language/tutorial/GridsRowsAndColumns.html