Kusari -Gama is on pg 145 under the Asian weapons table.
This is where your language barrier may come into play. Threat range is the number you need to in order to threaten a critical hit (look in the players handbook for critical hits and under the weapons section to see how threat range works). The areas you threaten are effectively the places in combat that you can attack with your current weapon (Read chapter 8 for this and it will probably make sense. If you still have questions I will try to explain it using different terminology). I highly suggest that if you have the PHB you look this up in your native language otherwise it may not make any sense as the two are so closely related as far as the word they use.
The reason I chose a reach weapon is that it theatens a larger area than the longsword. I wasn't worried about threat range. And the "strenght" of a weapon isn't usually determined by what catagory it falls under (light, medium, large, martial, ect) It has many factors including how you use it, what you are trying to accomplish with it, and a lot of other factors.