The GREGOR Infrared Spectrograph (GRIS) can perform spectropolarimetric observations in 1.0-1.8 μm and pure spectroscopic in the band 2.0-2.3 μm. The spectrograph has a Czerny-Turner design and has allowed the operation in the traditional long-slit mode since 2014. And also, in IFU mode since was upgraded in 2018. A third upgrade took place in 2021, where optomechanical, control and software improvements have been conducted to introduce a new spectral channel that receives light below 900 nm. This new spectral channel allows simultaneous observations with the previous infrared camera. Additional improvements include replacing the old diffraction grating with a new one and acquiring a sensor for the new spectral channel. The new grating allows having different order separation and maximising the combination of spectral bands of interest between both channels. Also, a dichroic was placed after the focal plane of the spectrograph, and a reimaging system was introduced in the new spectral channel to match the spatial scale (arcsec/px) in the old and new detectors. The feasibility of operating both cameras simultaneously, and synchronised with the polarisation modulation package, has been already tested. This work describes the optical design for all the spectral channels. We also examine its optical performance, as well as discussing future upgrades in terms of additional spectral channels that are already in progress.