First "Ground" is a misnomer; this is a name commonly given to the common power rail, which on the Pi, is rarely connected to "Ground".
ANY electrical circuit NEEDS to be complete. That means that you need a loop for current to flow.
Your proposed circuit has 2 loops;
- Pi GPIO, base resistor (missing), transistor Base, transistor
Emitter, and back to the Pi common rail.
- Battery, Load (missing), transistor Collector, transistor Emitter,
and back to the battery.
The first loop causes Base current to flow, which causes amplified current in the Collector. To complete both circuits they need to have a common connection.
Your circuit has a number of errors, and will NOT work. The load should be attached to the Collector, and Base current needs to be limited. See http://elinux.org/RPi_GPIO_Interface_Circuits
The circuit, as drawn will put ~2.6V across the load, the transistor will have 9-2.6V across it and dissipate almost 3 times the power delivered to the load! It will overheat the transistor and possibly destroy it, depending on the load current (not specified in the question). –