If you're cheap then just draw an alpha-blended poly over the screen *cough cough*

A quick nosey through the DX SDK showed up seemingly analagous stuff:

D3DGAMMARAMP (I'd imagine that would be prefixed with 'T', TD3DGammaRamp)
IDirect3DDevice8::GetGammaRamp
IDirect3DDevice8:etGammaRamp

[EDIT: Doh!] Looking at the wrong thing! Device calibration is totally unnecessary here. Ignore that quote!

Anyway, you'd probably use IDirect3DDevice8.GetDeviceCaps, passing in a TD3DCaps8 structure. You'd check that TD3DCaps8's caps2 for the flag D3DCAPS2_FULLSCREENGAMMA, plus probably some other caps. I've not used D3D so I can't say for sure.