It was designed as a separate unit for several reasons. But, it's part of the amp. Every 10^n comes with one.
One reason it's outside the box is so it can inhabit your pedalboard, where it really shines. Watch the instructional videos to see how it works.
It's a very versatile Gain-controlling device. It allows you to achieve distortion at very low volumes. This is probably it's most sought-after effect. You can play at flashlight volume and still achieve over-driven amp tone. For some, this is a dream come true, because they can't play loud in their home studios, but want a cranked amp tone, and the Gjika Gainmaster provides this ability, but only with Gjika amps. The exceptionally clean tone of Gjika amps is the key factor to the Gainmaster's utility, they go hand-in-hand.
The Gjika Gainmaster is a finely-tuned, engineered electronic device that employs specialized circuitry and a single tube - employed as a "floating front end" to the Gjika amps, they do not work the same with other amps. Bob repaired a vintage Marshall JMP 100 for a client, and used it in this video, to demonstrate how the Gainmaster sounds with his amp compared to the client's Marshall JMP 100.
This is why the Gjika Gainmaster is not sold separate from the 10^n amp.
The Gainmaster is ...Bob explains it in the video
More about the Gjika Gainmaster...
A Gjika Gainmaster "add-on"... 1-Transistor Germanium Fuzz Driver - Limited Supply NOS AC128 Vintage Transistors