Please, can we define, which GPU is used by the OpenGL ROP? I have tried to change the HOUDINI_OCL_DEVICENUMBER in houdini.env, but it seems, that this setting is not taken into account here.

No, unfortunately GL is not offered the same flexibility as CL by the OS in terms of specifying which device to use. The GPU used by the OS for graphics is the one all GL contexts are created on. There are some vendor specific extensions which might work, but they are somewhat complicated to use.
Vulkan fixed this, and allows apps to select the device from a list of devices. But while we're still using GL, that's not an option.

Thank you very much for your answer!