The first leaked pictures of the upcoming Sony FE 300mm f/2.8 GM OSS lens for Sony E-mount can be found here (licensed from Fotorunners). The lens development was announced back in January and the lens is already listed at B&H Photo.
Sony currently has the 600mm f/4 and the 400mm f/2.8 G Master lenses.
Sample photos from the new Sony FE 300mm f/2.8 GM OSS lens can be found here:
The new/upcoming Sony FE 300mm f/2.8 G Master OSS lens is already out in the field
Sony FE 300mm f/2.8 G Master OSS Lens Being Field Tested Now Plus Sample Images