The GB receivers are located on the mainboard and are R&S custom made BGA ASIC's. They are unobtainable, no datasheets available, so if your scope has this defect, then you have only a few options to get this fixed.
1) R&S standard repair, which costs around 5,000 USD.
2) try to obtain a second hand mainboard. I have been looking for one for more than 3 years, but with no luck. Nobody appears to have these spare on the shelf
3) If you are lucky, the problem is not with the ASIC's, but with the BGA soldering, so try to locate a shop, which can x-ray the mainboard to see if there are some suspect solder connections under the BGA's
4) if this is the case, you will need to source a company which can rework / reflow the board.
As far as option 4 is concerned, be careful which company you choose. I had my mainboard reworked, by a company, with as result that the board warped, lost a few 0201 resistors and/or capacitors.
Prior to sending the board for repair, Channel 1 & 2 were working fine, but after the "repair attempt none of the channels work anymore.
So be warned!