Opera still hasn't implemented back gesture yet, which is really unbelievable.

However, I found a quick solution that may be a good compromise: set three-fingers left/right gestures as Forward/Backward navigation in the Windows 10 touchpad settings. They're found in Settings > Devices > Touchpad > Advanced gesture configuration (visible if you have a compatible touchpad)

This doesn't solve the lacking two-fingers gesture, but I found that the gesture is really awesome system-wide (ex: Windows Explorer)