In the movies, the climb up and down ladders to get to them, the gunner positions obviously are on the top and bottom sides of the ship. However, when they are in the gunner postiion room, you can clearly see the passage/ladder behind the gunner seat. But they’re moving around, presumably parallel to the ground, like normal. Does gravity work differently in there? (Note, this behavior is the same whether they are in space or on a planet).
[waves hand in a vague, Jedi-like manner] “This is not the gravity you seek. Go home and reevalute your life choices.”
In other words, they never cared to have it make sense. It is what it is.
There is gravity in movie space. Also there is sound.
The gun turrets in the Falcon are supposed to evoke the image of ball turret gun positions in WW2 aircraft.
The gun turrets have their own, independent artificial gravity fields, separate from the one that provides gravity to the rest of the ship. This allows the gunner to rotate the gun through it’s whole field of motion without becoming dizzy or disoriented. Or falling out of their chair, since those thing apparently don’t have seat belts.
Speaking as a SW nerd: overthinking physics in Star Wars movies is a route to madness. 
To the OP: my assumption is that the artificial gravity in the dorsal gun turret is on the same plane as the rest of the Falcon, while the artifical gravity in the ventral turret is 180 degrees flipped from that of the rest of the ship. So, yes, climbing down into the ventral turret would have the gunner’s relative gravity “flip” as he or she went into the turret.
It looks like the dorsal turret (at least) is at 90 degrees to the rest of the ship - you can see that ladder that accesses the turret behind the gunners seat, with the ladder on “floor” relative to the person in the seat.
Like I said, probably the gravity field for the turrets is fixed to the “bottom” of the turret, on both guns. Even on the ventral turret, you’d want the ability to track the gun “up” through the full 180 degree arc, without falling out of your chair at the end because now you’re upside down.
Especially since despite the large number of computers they have, they target and fire exactly like a WW2 bomber gunner would. Not to mention who knows what the expected crew complement of the Falcon was. Kind of useless to have two turrets when the crew consists of only two beings.
Lucky for Han Luke was along for the ride - otherwise the fighters could just attack the side of the ship where he wasn’t.
Per the canon, it works fine as long as you don’t get cocky (kid).
Those turrets were definitely aftermarket additions. The baseline configuration of the Falcon’s model was designed to have minimal crew and maximum hauling space.
The Falcon was originally supposed to be what ended up being used as the Blocade Runner Tantive IV in the movie (because they didn’t want confusion with a similar ship in a contemporary movie). The design changed, the script didn’t. The Falcon in the movies is much too small to contain everything it was supposed to contain.
Maybe there’s a TARDIS in there somewhere.
The Falcon is basically the spacefaring equivalent of a semi truck. Those two big prongs on the front are supposed to hold a shipping container between them. Semi trucks don’t come with gun turrets, so those were definitely aftermarket.
I assume this is retconned right. I somewhat doubt they gave much thought about the inconsistency of a freighter having guns on it in 1977.
In the words of Harrison Ford / Han Solo, you shouldn’t worry about that because “it ain’t that kind of movie, kid.” Of course that seems to apply only to the original trilogy. Quotes like that do make me wonder how much of the character is actually Harrison Ford rather than whatever George Lucas wrote.
Though, Han was, from the start, established to be a smuggler, who regularly took on questionable cargos, and/or flew to questionable places. And, in the original film, he mentioned that he’d made “a few modifications” to the Falcon.
So, while a “typical” freighter in the Star Wars universe might not have guns, it makes sense for a smuggler’s ship to.
I’m pretty sure the original novelization of Star Wars back in 1977 (written by George Lucas himself Alan Dean Foster) actually did mention that the ship’s gravity in the quad-cannon turrets was set up to be perpendicular to the artificial gravity field in the rest of the ship.
But the coal rolling comes factory.
In space opera, the space freighters have to deal with space pirates. Therefore, guns.
Fun fact!
Established in the Han Solo novels by Brian Daley…
In the absence of a third crew member to pilot, when encountering hostile ships, they’d put it on autopilot and both take a gun. Han and Chewbacca had a standing bet for each ship destroyed. In addition, there was the ‘money zone’. That’s the space along the edge of the ship where the fields of fire overlapped. A kill there counted double in the betting system.
Those are good, terrible books and I encourage you to read them.
“You PAY to see the cards, Gallandro. You folded.”
One would think “auto gunning” would be easier to pull off than advanced evasion techniques.