In order to determine which value to write to each register, you'll need to use the appropriate table that explains what the bits in the register do so you can build a value that represents all of the settings you want.
For example, we can look at Tables 19, 20, and 21 inside the gyro datasheet, which describe the CTRL_REG1 register. Values assigned to the DR1, DR0, BW1, BW0 bits determine the output data rate (ODR) and frequency cut-off as described in Table 21. So, if you are interested in an ODR of 95 Hz and a cut-off frequency of 12.5 Hz, then DR1, DR0, BW1, and BW0 should be set to 0, 0, 0, and 0, respectively. Then, you can continue to set bits that correspond to the behavior you are interested in. Let's complete the example by putting the gyro in normal power mode and enabling all axes. To do this, you would want to write a value that corresponds to "00000111":
DR1 DR0 BW1 BW0 PD Zen Xen Yen
0 0 0 0 0 1 1 1
"00000111" is 0x07 in hexadecimal, or 7 in decimal. All of those values are valid for the write function:
gyro.writeReg(L3G_CTRL_REG1, 0x07); // hexadecimal version
gyro.writeReg(L3G_CTRL_REG1, 0b00000111); // binary version
gyro.writeReg(L3G_CTRL_REG1, 7); // decimal version