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As a work of Justin Roiland, creator of Rick and Morty, High on Life is generally a rather... silly affair. It's absolutely jam-packed with random references, bodily excretions, wanton murder, and at least one alien with a farting butt for a head. However, though his works may be silly on the surface, it pays to remember that Justin Roiland knows a thing or two about building a mythos.
High on Life is jam-packed with lore regarding the way its version of the universe and its various peoples live their lives, and perhaps some of the most interesting bits of lore revolve around your trusty sidearms, the Gatlians. So, who exactly are the Gatlians, and how did they end up in the hands (or hand-adjacent appendages) of the G3 Cartel?
Warning: Spoilers for High on Life ahead
Gatlus, Home Of The Gatlians
At an indeterminate point in the past, the Gatlians all lived on their home planet of Gatlus. Gatlus was not unlike the planet Earth; it was located on the very furthest boundaries of the universe, well outside the view of any spacefaring societies. While it was a fairly advanced planet with a similar level of technology to our own, none of its residents seemed to have much interest in the realms beyond the stars. None of them that is, except for Kenny.
Kenny was an old-school dreamer. He didn't have much going for him on his home planet, with his interests instead being in the wonders of outer space. He wished with all his heart that he could one day leave his home and strike out on his own. His wish was granted in the form of a crashed spaceship owned by one Rel Delmar. A smuggler by trade, Delmar had been just about anywhere and everywhere in the known universe, as well as the unknown, given his sudden arrival on Gatlus. He had never seen a living weapon like Kenny before and took an immediate interest in him. After the two repaired Delmar's ship, he asked Kenny to join him as a member of his crew and his personal firearm. Kenny accepted with no hesitation.
How The G3 Found The Gatlians
For a time, things were nice. Kenny was finally free of Gatlus and exploring the stars as he had wanted. He got to meet new people, experience new cultures, and of course, shoot a lot of stuff. However, as a member of a crew of smugglers, Kenny often found himself in the company of shady characters, and eventually, he came into contact with the shadiest of them all: Garmantuous, leader of the G3 Cartel.
Garmantuous, ever the enthusiast of the rare and unknown, was fascinated with Kenny's abilities as a firearm, from his infinitely firing shots to his bizarre Glob Shot ability. If the G3 could arm themselves with such weapons, they'd be unstoppable. Under the veil of simple curiosity, Garmantuous asked where Kenny had come from, and Kenny, ever the naive optimist, led him right back to Gatlus. Presumably, Kenny expected a peaceful first contact. Instead, he got an all-out war.
What Happened To Gatlus
The G3 immediately launched an offensive on the planet in an effort to subjugate and conscript the Gatlians to their cause. As natural combatants, the Gatlians fought back; they actually did a fairly good job of repelling the G3's forces. With a straightforward war off the table, Garmantuous turned to more devious methods. Establishing contact with one Dr. Gurgula, an outlaw scientist, Garmantuous commissioned the creation of a world-destroying bio-weapon. Gurgula, eager to test a few hypotheses, engineered a zombie plague on Gatlus, wiping out the vast majority of the Gatlian species.
The precious few immune to the virus were summarily overrun by the G3. In the last moments before their total defeat, Kenny confided in his friend and mentor, Lezduit, right before he (supposedly) passed away. All the remaining Gatlians were taken away by the G3 to serve out their days as G3 weapons, while the remains of Gatlus were abandoned, completely destroyed, and utterly inhospitable.
G3 Performed Experiments On The Gatlians
With the Gatlians firmly under their metaphorical thumbs, the G3 got to work engaging in some seriously unethical science to better mold them to their needs. For starters, many Gatlians, including Kenny and your other guns, were fitted with Inhibitor Chips that allowed G3 members to use their firepower regardless of the Gatlians' consent. Most of the G3's highest officers were given Gatlians modified in this way, while the lower Merks of their forces had Gatlians in a vegetative state physically bonded to their arms to let them use them.
After that, more radical experiments began. One of your Gatlians, Creature, was a completely different person before the G3's scientists got their hands on him. His memory was completely wiped, and his body and brain were heavily modified to perfect his ability to launch his own progeny. Even the dead weren't immune to this cruelty, as the remains of Lezduit were recovered and cybernetically enhanced to utilize his powerful offensive abilities.
This was the lives of the Gatlian race for an unknown stretch of time, at least until the G3 happened to stumble upon planet Earth and, in the process of raiding it, dropped Kenny for you to pick up. With the Bounty Hunter's help, all the Gatlians held by the top officers were liberated, but it was a pyrrhic victory. It is possible, and highly likely, that Kenny, Gus, Sweezy, Creature, and Lezduit are the last living Gatlians in the entire universe.