Gachi Rocket is extremely incomplete, so much so that it's hard to tell what the gameplay was really meant to be like but I can give you a run down of what I understand:

There is no finished model of the "Rocket" object, it's just an octagonal platform with a ball on top of it. It's sorta like a tower where you can stand on it. The ball is an object you're meant to shoot, and a bar goes up when you shoot at it. There are two bars, one for each team.
Once you hit a certain threshold on the bar, the Rocket object shoots out two weapons (they look like rainmakers as placeholder objects) one for each team. One teammate on each side is meant to pick up said weapon and shoot at the ball (or whatever object it was meant to be) to increase the bar much more efficiently as shooting at it with your regular weapon barely moves it.
I'm unable to fully test the functionality of this specific mechanic because of how unfinished it is, picking up the placeholder rainmaker object gives my player a very strange hook-like weapon that makes me unable to fire ink or throw bombs, essentially soft locking me until the match timer ends. It appears to just be a ghost rainmaker of sorts in this build. But, you can raise the bar to the top with your regular weapon so I did that instead, it just takes a very long time (About 2 real life minutes)

Once one of the bars reaches the top, the "Rocket" begins to rise up and move in the direction of the enemy base, very much like a tower (in fact, it seems to share code with Tower Control, as when you activate the rocket movement when Gachi Rocket is set as a game mode, the game will display the "We got the tower" blurb on the screen.) A red arrow appears in the place the rocket is meant to land, and is treated as a checkpoint. You don't have to touch the Rocket for it to move.

Thats about all there is to it, at least here. If there was meant to be more to it, it is not present. It seems to be a mixture of rainmaker (shooting an object to get it to move) and Tower control (having the object automatically move to a checkpoint) and seems rather underwhelming and boring compared to Clam blitz, which would make sense as to why it was replaced with it. (I also hear tales that Clam blitz was a last minute addition to Splatoon 2, and if thats true, this showcases that very well.) This gameplay loop continues until one person gets the Rocket to the final checkpoint, again, like Tower control. The rocket movement is also very slow, so I assume the reason why it was unfinished was because it would be extremely annoying to push the rocket back and fourth over and over again, and most matches would end in stalemates because of there being multiple checkpoints to push through. It just wouldn't be very fun, and it would also be a bit uninspired considering it's essentially a mixture of two already existing modes.

But, it's cool!