The best choice for the first analysis of this problem might be OptiBPM. It is a diffused planar waveguide, so OptiBPM has the most tools for dealing with the waveguide. The BP method can be used for long period gratings, because the propagation is paraxial and the coupling is two co-propagating waves. OptiBPM will be accurate but computationally intensive, especially to create a spectrum from this. OptiBPM must simulate one wavelength at a time.
To create a transmission spectrum, it would be best to use OptiGrating in the Other Waveguide mode. Then use results from OptiBPM (overlap integral, modal index) for the input required in the Mode Properties dialog box of OptiGrating.