Download page says that Whonix GW is experimentally supported on Raspberry Pi 3 B, is it compatible with B+ model?
iirc back then when I created the scripts it was not. Should be possible now according to: https://wiki.debian.org/RaspberryPiImages
Easiest would be to give it a try and report back. Maybe there would need to be some changes to the build script or boot partition.