You can turn on Branched Path Tracing and try how it works, it's not disabled or anything. You should however keep the individual sample settings to 1 (all the ones on the left side), to prevent the GPU process from getting a time out. If the scene and shaders are too complex however, even low settings cannot prevent the OS from interrupting the GPU render process, and that's why the UI recommends to not use GPU and branched path tracing together.