As far as I can gather Grim Tales is mostly the same as d20 modern, with some other bits in. Beyond the basics d20 modern and D&D are fairly different.
Grim Tales is a toolkit, not a setting. Grim Tales is a lot of d20 modern, with bits of D&D put back in (IE: the D&D core class abilities have been added as d20 modern style 'talents' and put back into GT).
