It is possible to develop firmware for BlueNRG-2 on System Workbench for STM32?
For BlueNRG1 or BlueNRG2, ST provides their own SDK with close sourced BLE lib. You should download and install the SDK to start experimental project.
There are example projects in the SDK packages. Those projects are for IAR/MSD/Atollic and all on Windows. Although the Atollic TrueStudio can run on Linux and MacOS, the example project itself has some settings which only for Windows, so has to fix errors before can build Atollic example project on Linux/MacOS. I have those fix available, and can push to github if some demands exist.
I had questioned ST, as they said, there no plan to support BlueNRG1/2 on STM32CubeIDE. So right now Atollic TrueStudio is the only hope.