I looked up GZ250 and it's an air-cooled bike, like mine, also a 250. Some bikes have fans, some bikes have coolant inside, but ours just air it out. Because of this getting caught in traffic can easily lead to overheating esp after coming off high RPM, highway driving. However, in your case, it sounds like you think something is not right.
How do you know that it's overheating? Do you smell something or do you have a temperature gauge that goes into red? Is it the engine that's overheating? Did the leaking of the oil coincide with the overheating?
I'm asking all these questions because it would help determine things.