A function to extract graphics primitives:
ClearAll[getVrtxCoords];
getVrtxCoords[plot_, prims : {___}] := Module[{pts, pts2,
prms = Alternatives @@ prims,
vcoords = Cases[plot, GraphicsComplex[{coords___}, __] :> {coords}, {0, Infinity}]},
pts = Cases[plot, prms[points : {({_, _} | {_, _, _}) ..}, ___] :> points, {0,
Infinity}];
If[vcoords === {},
If[pts === {}, {}, First@pts],
If[(pts2 = Cases[plot, prms[{points___}, ___] :>
((First[vcoords][[#]]) & /@ {points}), {0, Infinity}]) == {}, {}, First@pts2]]]
Examples:
(* OP's example : *)
p = {{0, 0, 0}, {0, 0, 300}, {0, 300, 300}, {0, 300, 0}};
myGraphics = Graphics3D[{Polygon[p]}, Boxed -> False, Lighting -> {Gray}];
Grid[{{"image", "Points", "Lines", "Polygons"},
Prepend[Column[N[getVrtxCoords[myGraphics, {#}]]] & /@
{Point, Line, Polygon}, myGraphics]}, Spacings -> {1, 1}, Dividers -> All]

A 2D
example with GraphicsComplex
:
v = Table[15 {Cos[t], Sin[t]}, {t, 0, 4 Pi, 4 Pi/5}];
grphcs2 = Graphics[GraphicsComplex[v, {Red, Thick, Polygon[{1, 2, 3, 4, 5, 6}],
Blue, Opacity[.7], Polygon[{{1, 2, 3}, {3, 4, 5}},
VertexColors -> {{Yellow, Yellow, Yellow}, {Blue, Blue, Blue}}],
Opacity[1], Dashed, Thickness[.02], Brown, Line[{1, 4, 2}],
PointSize[Large], Red, Point[{1, 2, 3, 4, 5}]}]];
Grid[{{"image", "Points", "Lines", "Polygons"},
Prepend[Column[N[getVrtxCoords[grphcs2, {#}]]] & /@
{Point, Line, Polygon}, grphcs2]}, Spacings -> {1, 1}, Dividers -> All]

A 3D
example:
v3 = PolyhedronData["Dodecahedron", "VertexCoordinates"];
Short[i = PolyhedronData["Dodecahedron", "FaceIndices"]];
dodec = Graphics3D[{Yellow, GraphicsComplex[v3, Polygon[i]]}, ImageSize -> 300];
modifieddodec = Graphics3D[{Opacity[.8], EdgeForm[Opacity[.3]],
Polygon[#, VertexColors -> Table[Hue[RandomReal[]], {25}]] & /@
getVrtxCoords[dodec, {Polygon}]}, Lighting -> "Neutral", ImageSize -> 300];
Grid[{{"image", "reconstructed", "Points", "Lines", "Polygons"},
{dodec, modifieddodec, N@getVrtxCoords[dodec, {Point}],
N@getVrtxCoords[dodec, {Line}],
Short[N@getVrtxCoords[dodec, {Polygon}], 10]}}, Spacings -> {1, 1},
Dividers -> All, Alignment -> {Center, Top}]

myGraphics
? If so, tryInputForm@myGraphics
. $\endgroup$by clicking the checkmark sign
` $\endgroup$