Gyro - Determine angle accuracy from datasheet
Well, first you go to section 6.1, Gyroscope Specifications. There you will see that the zero reading can be as much as 20 degrees per second, and that it can vary as much as 20 degrees/second over the rated temperature range. If you can run a zero point reference cycle just before your measurement, you can compensate for both of those. If not, the best you can do is a calibration procedure to determine the offset for various temperatures, then measure the temperature of the unit at the time of measurement, and use that as a correction factor. Fortunately, the unit also measures temperature, so that's simple enough.

Second, you notice that scale factor has a tolerance, so you'll need to calibrate each unit if your desired accuracy is less than the tolerance. Ditto for temperature effects.

Then you notice that there is a noise specification as well, so you take that into account, usually by filtering the data to reduce total noise.

Oh yes, and depending on your requirements, you'll also need to analyze your system to determine the power supply stability, since that also affects the output. 

In other words, in order to "work out how accurate it will be at calculating angels", first you need to understand the voltage and temperature behavior of your measurement system, then determine exactly how you are going to acquire and process your data. Then apply that information to the specifications in the data sheet.