That supercharger has been there for quite a while. I remember seeing it there during the early drone over-flights. Perhaps the cables are below the gravel, but I am quite sure it is functional. From what I understand is this supercharger is for Tesla executives and employees, therefore not available to the public and problably never will be. But I am sure Tesla will build a fully functional supercharger site available to the public at the gigafactory site. Wouldn't be surprised if they also have a parking lot of standard L2 HPWCs around too.
Tesla has a lot of people traveling between the Bay Area and Nevada right now. I've read stories of people here on the forum often meet Tesla employees on I-80 at SCs. I expect the SC at the Gigafactory is to charge the company vehicles going back and forth.
That portable Supercharger is similar to the ones at Harris Ranch and has been noted in Gigagfactory photos from months ago. It is well inside the Gigafactory property so not open to the public. Obviously for use by Tesla employees. I assume that in a year or less there will be a permanent Supercharger on that site, again for internal use only.