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Odometry of the robot basically means where the robot thinks it is relative to where it started from. Now, odometry can be provided by a variety of sources - encoders in the wheels, cameras (visual odometry) or even laser scans.

So now, you need a way to provide odometry to your gmapping node. And since the only source of information that you have are the network packets from your Velodyne, you will have to write a lidar based odometry node. You can use ICP to try and match consecutive scans, and find out the movement of the robot between these scans (change in pose) and then broadcast that on the /tf topic as the transform between /odom and /laser.

Feed that in to gmapping and you're good.