Does rooting phone wipe your data if the bootloader is unlocked?

I'm not familiar with the G2 specifically, but in general, no, rooting a device on its own should not wipe data. Rooting your phone is the process of getting the su binary installed and available to access root privileges. That said, @SarpSTA is right in stating that a factory reset shouldn't unroot your phone. The su binary is placed in the system partition, which is left intact on wipe. If a wipe touched /system, your phone would be bricked and unusable, requiring a low-level flash to recover. What you may be missing is an app, such as SuperSU, to control access to root.