This node is best viewed in a font which clearly distinguishes lowercase Ll and capital Ii.
SiIvaGunner—pronounced /ˈ siː.vəˌ gə.nɚ/—is a YouTube channel hosting the works of a vast collective of remix artists, whose primary output is mashups of video game music with other things, jocularly referred to as "high quality video game rips" and presented in a way so as to appear as unaltered video game music.
Those of you that are already fans, may be asking, "Why would you just come out and say it? You're spoiling the joke!" And yes, I'm giving the game away. You see, the capital I in SiIva looks very much like a lowercase L in YouTube's fonts, and this is a reference to SilvaGunner, a YouTube channel that actually posts unaltered high quality video game music. But in actual fact, it's been so long since this duplicity was actually accurate or effective, and the 'secret' is so open, that at this point the altered nature of the so-called high quality rips is actually the selling point.
Here, let me give you some history. In 2008, a YouTube channel called SilvaGunner started uploading video game music, until it was removed for copyright violations in 2010. Shortly thereafter, it was revived under a new name, the spoonerism GilvaSunner, and there the project has remained relatively unmolested by the powers that be. In January 2016 began our channel with the false rips, under the name GiIvaSunner, and it attempted in earnest to be a false version of GilvaSunner, bearing the same avatar, video title format, and metadata. It survived for a paltry four months before removal, and came back in the form we know it today, SiIvaGunner.
So what do these altered rips look like? Well, in the beginning, they were very faithful renditions of the video game music they were masquerading as, at least for the first minute or so. Then, they would subtly transition into playing a different theme, most famously the opening melody to The Flintstones, while still keeping in the same soundfont as the track it began as. A simple and effective formula, requiring a nontrivial amount of digital audio prowess. The bait-and-switch format was of course not invented by SiIvaGunner—mashup artists have been playing with this technique for years—but they definitely codified it.
As the channel grew in popularity, and people caught on, it became a game of guessing or at least catching "the reference". SiIva started having the freedom to make more obscure references, and more importantly, to troll its fanbase with ever more obnoxious references: some channel defining classics include (but are to no extent limited to) the Flintstones theme, Snow Halation from multimedia series Love Live!, U Guessed It by rapper OG Maco, and the opening theme to adult cartoon television series The Nutshack. It developed into a channel with an underlying story arc, sometimes fully textually as a "channel takeover" by some fictional villain or other event, sometimes metatextually as SiIva vs his channel's viewers.
It should not be understated that SiIvaGunner is a collective effort. A large number of "rippers"—mainly internet remix artists on YouTube and SoundCloud that were particularly tuned into contemporary meme culture—worked together, initially in secret, to create rips for the channel. This enabled it to publish rips with astounding frequency and consistency, as often as once per hour every day. A small number of them were low effort shitposts, but most were competently executed musical bait-and-switches, and a fair few went above and beyond to create so called "actually good" mashups, sometimes as multi-artist collaborations, with accompanying videos of equally high effort.
And they were a vigilant bunch with time on their hands. If their doppleganger GilvaSunner started posting a lot of music from some particular game, they would quickly incorporate it into their sources. If there was a much-anticipated video game announcement on the horizon, they would be preparing for it. If the viewers were responding especially positively or negatively to some new perceived channel trend, they would waste no time in leaning into it.
This gave the channel a mystical quality, because the consistency of its outward presentation and the childish nature of the content contrasted heavily with the prolific rate of video publishing. The group authorship was a rather well-kept secret for a long time. Eventually, details would slowly slip out as the standards of secrecy grew more lax, but by then the phenomenon was already self-sustaining, and the channel itself still preserved the kayfabe.
Then, after another five months, SiIvaGunner announced the project would be ending. It would return a few more times, for Halloween and Christmas, but ending nonetheless. This kicked off a chain of hiatuses, comebacks, cancellations and uncancellations and cancellation scares which has resulted in a number of "endings" and distinct "seasons" or "reboots" to the channel, but ultimately it remains posting rips to this day.
It is hard to give generic examples of SiIvaGunner content, because the intended mode of interaction (at least for the casual fans) is for the viewer to click on a video containing video game music they are familiar with, and then have those expectations subverted. Sure, there's a small chance that you might luck out and still get the reference even despite not recognizing the game music. But if you don't play a lot of games or know a lot of memes, then in all likelihood SiIva will sail right by you and you may not feel it is as remarkable as someone more familiar with it might feel.
So what I'll do is this: I'll give a link to the channel, and urge you to peruse a bit of it yourself. Try the Popular Uploads page, or browse the playlists which sort the rips by their original game. But as an alternative to that level of effort, here are some of my favourite rips, listed by video title and by the swapped-in reference. Maybe they will speak to you too.
Finally, I would be remiss not to mention the lasting legacy. The formula was immensely popular, and soon after SiIva got big (but especially during its many hiatuses) spinoffs, copycats, and variants popped up. Everybody wanted their own take on the matter, either leaning harder into the channel's memes than SiIva was going to, or just wanting a lower threshhold for quality (the shitpost principle), or even to expand the purview from games to other kinds of music. The three most resilient descendants I am thinking of fit neatly into these three categories: they are TimmyTurnersGrandDad (punnily combining TimmyTurnersDad, the user behind GilvaSunner, with beloved SiIva meme "7 Grand Dad"), VvvvvaVvvvvr (a satire of SiIvaGunner fan channels), and recordcollector1972 (the bait-and-switch formula applied to pop and music-snob music alike), respectively. If the formula intrigues you, these are also worth a perusal. (rc1972 is a particular favourite of mine.)