I'll assume you have a set of time domain (rise time, settling time, overshoot etc) or frequency domain (gain margin, phase margin etc) criteria that should be met by your closed loop system. Now you have multiple options to estimate and tune your PI gains.
1) Use root locus technique to see the movement of system Poles-Zeros in the S-Domain and pick the gain which meets your desired criteria.
2) Use MATLAB pidTuner. Its a very user friendly tool to tune your controller. Just type 'pidTuner' in your command prompt. Contrary to the name, you have option to choose type of controller (P,PI,PID etc).
Hope this helps. Get back in case you are having any difficulty.