First here's the datasheet
http://www.allegromicro.com/datafile/0642.pdf
Try to think of it this way......
When the gear is moving forward (CW), as a tooth passes, the digital signal from the GTS goes high (0 to 1 or off to on) and stays high for a specified length of time and then turns off.
When the gear is moving backward(CCW), as a tooth passes, the digital signal from the GTS goes high (0 to 1 or off to on) and stays high for a specified length of time and then turns off.
What is different between the two is the length of the time that the signal remains high. So besides noting the interrupt or the transition from 0 to 1, you would now also need to know the length of time it remained one.
Thus by knowing the time the signal reamained one, based on the datasheet you would know the direction. There is a time range for sensing forward, and a different time range for reverse. Last years sensor was 45microseconds(CW) for forward and 90microseconds(CCW) for reverse.
I hope this helps.