The software that this Gateway adapter cable enables is Tesla Powertrain Diagnostics (TPD). Info on obtaining and installing TPD is
here. It's widely available in the original v1.1.4
2 as well as the newer and slightly less capable 1.1.4
6 . The latter is, however, much easier to install and maintain.
Using it is a matter of poking at it and seeing what it does. It's mostly valuable to see Alerts (what Tesla calles DTCs), to see the spot values of various items (eg how the DC-DC Converter is working, what the 12v battery voltage is, is the OBC able to detect CP correctly, what is the state of the HV Contactors), but it does have a few tools available, such as the ability to run the three coolant pumps at 100% for ~33 minutes to purge air from the two cooling systems.
There is no Help manual for using it, so you have to blunder about. There's a Help button, but it shows you how to do things like prepare a log to upload to Corporate for review, not how to use the software as a tech.
The adapter cable connects to the second OBDII-like connector, located at the left rear of the RAV4 EV, behind an access panel. It's referred to as "Diagnostic Link Connector" DLC3 No. 2 in the Toyota docs, to distinguish it from the typical OBDII connector under the steering wheel area.
To work with the Toyota systems, you use the front connector + Toyota Techstream software and a J2534 dongle cable.
To work with the Tesla systems, you use the rear connector + TPD and this adapter cable, which enables communication with the Tesla Gateway module.