Rare. Here's the info from Pong-Story.com:  Heathkit's GD-1999 is a strange PONG system. Although Heathkit was famous for selling high quality products, the GD-1999 was not a real kit. As a matter of fact, this system was a clone of National Semiconductor's "Adversary" system (model 370, released after NS launched the MM-57100 game chip in 1976). The system is same as the Adversary with one exception: the label says "Heathkit GD-1999" instead of "Adversary", and the model passed from 370 to GD-1999.

Another reason for not being a real kit is the electronic circuit board. Rather than being hand soldered, it came fully assembled, adjusted and tested. Only a few solders were required to wire the controllers to the circuit. The rest was really simple: screwing the two parts of the case after putting the circuit inside. The user manual shows quite well the ease of assembling: only four double-sided A4 pages instead of the thick 100-page manual full of illustrations and details like of the GD-1380, which was a true kit.

Talking about the electronic circuit board provided by National Semiconductor, another obscure system (supposedly a kit) was made by GEMINI (model 7640) and used the same circuit. If the Gemini 7640 was released in late 1976, the GD-1999 was released in 1977 and sold at a very low price. Curiously, not many sold (maybe a few thousands or less)