Hi,
The gps won't really help for the drift problem at local scale. Are you using ROS or the standalone app? Which RTAB-Map version? If you have a database to share, it could help seeing what kind of environment you are mapping. If it is outdoor and there could be direct sunlight, I recommend to use the zed camera over the kinect.
If you have some money to spend to get a very simple scanner with low drift, you may look at the Google Tango option (e.g., Asus Zenfone AR), with
RTAB-Map Tango.
cheers,
Mathieu