Each individual GBI module on the Genius bus requires a unique serial bus address (SBA). You can't have two GBI modules with SBA 20 on the bus at the same time.
The actual range for SBA is from zero to 31, but SBA 31 is reserved for the bus controller (the BEM331).
The following is a historical footnote, and doesn't pertain to your issues.
The limitation of SBA from 16 to 23 dates back to the earliest days of Genius on the 90-30. In the initial implementation in the early 1990s, there was no bus controller for the 90-30. All that was available was a Genius communications module which was limited to communication with SBAs 16 through 23. I was told by a GE/Fanuc marketing manager that the reason for limiting Genius capabilities in the 90-30 was not due to any technical issues. The true reason was to drive sales of the 90-70 product line which, as the successor to the Series Six, had full Genius capabilities and was targeted at the higher end of the PLC market. In his words, "If we packed everything into the 90-30, we'd never sell any 90-70s."