GS150 will give an average of 28-30km in a liter if ridden above 80kmph and 40km if ridden between 50 to 80kmph.
CG125 will give an average of 35km in a litre if ridden above 80kmph and 45 if ridden between 50 to 80kmph.
Handling & comfort:
GS150 is very smooth when it comes to shock absorbers but the brakes and grip are not very good . You'll have to put a thicker tire in the rear to make it stop on braking.
CG125 is not very smooth but the brakes and grip are good.
GS150 has a 145cc OHC engine with 5 gears transmission which gives very smooth ride experience and is very good for long rides. OHC Engines are not very robust and need frequent maintenance.
CG125 has a 124cc OHV engine with 4 gear transmission and probably not very good for long rides. But the OHV engines are very robust and won't need frequent maintenance.
GS150 will easily do 110-115kmph
CG125 will easily do 100-105kmph
If you are willing to buy a GS150 just put a thicker tire on the rear and you'll be a very happy rider. And if you can sacrifice some smoothness and power then CG125 is there. I ride both and i love them both.