When initially created, the forward transformation of a GrismMap transforms input "grism parameter" values into output wavelength values. The "grism parameter" is a dimensionless value which is linearly related to position on the detector. It is defined in FITS-WCS paper III as "the offset on the detector from the point of intersection of the camera axis, measured in units of the effective local length". The units in which wavelength values are expected or returned is determined by the values supplied for the GrismWaveR, GrismNRP and GrismG attribute: whatever units are used for these attributes will also be used for the wavelength values.
AST A Library for Handling World Coordinate Systems in Astronomy