You can get the cut sheet with some details at the Wayback Machine:
The GCR was made for BP by Steca, I haven't seen one in a long time, but if IIRC it is pretty similar to the Steca PR (at least the GCR/M with display is) so maybe that manual
will be similar? You might have a look through the Steca archive
page for other models that might be close since it was a re-badged version of something they built.