I agree with Bob (for once ). It does not need the apostrophe because it is not possessive. It is just more than one G and less than three Gs.
Gene suggested 2G 3D since that is what we do. So I tried a solution with that in mind. It works but I always prefer simple.
I also tried a very elegant 2 (Poster Bodoni MT) which plays nicely with the simplicity of the D shape.
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