I have multiple drones work in swarm formation, i made the quadcopter model and the swarm one. Until now i have the swarm moving in a formation leader-follower and track a predefined trajectory based on PID Controller. What i need help is how to add GPS, IMU sensors model to my model?
It depends on what you are trying to achieve.
If you want to see how your path following / formation tracking etc. works under non-ideal conditions, then you can simply give it a position that is slightly wrong. Perhaps some kind of Gaussian around the actual location. It is kind of pointless to be in a simulation environment with perfect localization, then simulate multiple noisy sensors, then use a complicated algorithm to fuse the sensors to try to get back out good localization. Why not just simulate the output of all that mess? i.e. "good" not "perfect" localization.
To reiterate, it is totally reasonable to not want to use perfect localization in your simulator. But there are easier ways to go about it.
If you want to make your simulation more closely match your real hardware, I would highly recommend looking at the noise, error, and other characteristics of your actual sensors, then adding those to the simulator. Every sensor, platform, and application is slightly different, so it will be difficult for anyone else to provide this data for you.
But in general, from what I've seen, GPS usually has some constant offset from the actual position, then in a little while, it will jump to some other offset. Acclerometers usually have a scale error, or axis misalignment. And gyros have drift.
If you want to test out your kalman filter or whatever sensor fusion algorithm you have, then that is probably better served in a different question.