is modifying the exhaust pipe into open pipe could change gas consumption?
 

i own a 125 cc 4 stroke motorbike.

It will but you get good mpg with a 125 cc 4 stroke motorbike so why worry.

Yes, it will have been set up to get the best possible so expect to get less.

Lower fuel consumption, more acceleration, lower top speed. noise increase. more attention from the police.

not noticably. but the powerband might change just a bit.

In fitting an open exhaust you will push the usable power up the rev range, decrees your engine low down toque as you are reducing the back pressure but will get an increase in top end power! Your fuel consumption will rise as you have to keep the revs higher at low down speeds as you have lost the torque.

As the breathing system of any engine is deigned as a unit i.e. the flow is carefully developed from intake to exhaust it is probable that your bike will run less efficiently with an open exhaust unless you mofify carburation to compensate...not an easy task for the amateur. Furthermore you will render your bike illegal for road use and you will probably make your neighbours very anti-motorcyclist. I love motorcycling but I am very much against straight through exhausts on any road vehicle....for 2 strokes read "hate" for just being "against"