yes the time quantum is 2.Since a process is alotted this fixed time, and after 2ms(or whatever the time unit is), process P2 has arrived in the queue, so the scheduler will preempt P1 and push it in the queue after process P2, and process P2 will run for the given time quantum. The ready queue for first 4 time units will look as :
P1 P2 P1.... when P2 runs P3 arrives before its alloted time has finished...and the queue will be :
P1 P2 P1 P3.
Hence shouldn't the gantt chart be like this :