1- do you mean to maintain a constant PWM so that my speed will be constant?
Yes, IF all external forces remain constant. However, that is almost never the case which is why feed-back is so commonly used. With feed-back the velocity can be held constant by changing the PWM duty to compensate for changes in external forces (which can also be changes in items like gear box friction and motor efficiency).
2- is their any way to simulate this model, using MATLAB Simulink, or other software , or it will be hard as it depends on the motor type and specifications??
It can be modeled but it is difficult due to the reasons you stated. You can make some assumptions and guesses for an approximate model that would be helpful to work out how to do it.
I don't know how to obtain a relation ship between them or how to put my assumptions on?
The beauty of feed-back control is that the system can be self correcting within reason. You could also build parts of the system and make measurements to have more than just some guesses.
A place to start is the DC specs of the gear motors. Assume a 100% duty PWM (DC) and the no load RPM. This would be the absolute top speed. Then assume some loading that will slow the motor with 100% PWM (here guess or measure a real motor).
Now assume or measure the loaded speed at 50% PWM and set the target (desired) speed for this RPM.
Choose an encoder configuration, number of ticks per revolution, and calculate the relation between the encoder and the PWM duty.
Now you have a basic model. If you can then make measurements of a real motor/gear box/sheel & Bot you can refine the model.