Unfortunately, diffusion gradients are typically vendor specific in DICOM. Each vendor using their own tags and coordinate system to define the gradients. The reason dcm2niix does not support Toshiba gradients is that I have not seen a Toshiba diffusion dataset. I do not have access to Toshiba hardware, so I rely on the community to provide sample datasets. 

Ideally, it would be great to have a publicly shareable dataset that illustrates Toshiba gradients. In addition, it would be great if the dataset included two series: one acquired orthogonal to the scanner bore and one rotated by about 30 degrees. The rationale (along with sample datasets from other vendors) is described in the "dedicated document" linked here
  https://www.nitrc.org/plugins/mwiki/index.php/dcm2nii:MainPage#Diffusion_Tensor_Imaging