How does the spell Reverse Gravity affect creatures with spider climbing?

Question

I understand that such a question is rather left up to the DM. However that can come as a situation "DM against player" (which is not good but sometimes inevitable). So I'd like to know your interpretations and avoid being prejudiced.

Reverse Gravity (emphasis added):

This spell reverses gravity in a 50-foot-radius, 100-foot high cylinder centered on a point within range. All creatures and objects that aren’t somehow anchored to the ground in the area fall upward and reach the top of the area when you cast this spell. A creature can make a Dexterity saving throw to grab onto a fixed object it can reach, thus avoiding the fall. [...]

As I understood from the description, the spell changes the laws of physics in a certain area and causes objects and creatures in it to fall on the ceiling as if it were a floor.

Now, a creature with spider climb ability can climb difficult surfaces, including upside-down on ceilings, without needing to make an ability check.

I found two possible logical conclusions:

1st: While creatures, such as Giant Spiders, have spider climb and a climbing speed, they still have their normal movement speed, which is logical: while they are moving on the floor they are walking/standing. That makes them equal to all other normal walkers and the spell affects them the same as everyone.

2nd: The moment that the floor becomes ceiling, these creatures can instantly attach to it and become anchored to "the ground". After all, no actions needed to activate the spider climb as a creature walks from the wall to the ceiling. That's why spell doesn't affect them unless they are incapacitated/prone etc.

Answer 1

It doesn't affect them.

If you are affected with spider climb, you can treat any difficult or impossible surface that you would normally need to climb, as normal flooring.

Monster ability:

Spider Climb. The spider can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Spell:

Until the spell ends, one willing creature you touch gains the ability to move up, down, and across vertical surfaces and upside down along ceilings, while leaving its hands free. The target also gains a climbing speed equal to its walking speed.

Rules on climbing:

While climbing or Swimming, each foot of Movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a climbing or Swimming speed. At the GM’s option, climbing a slippery vertical surface or one with few handholds requires a successful Strength (Athletics) check.

The monster ability negates the ability checks you might need to climb a surface, and the spell negates one that might trigger an ability check: [surface] with few handholds, since you are not using your hands to climb, you walk on the surface.

There is no distinct definition of "Anchored". But since there's no need to "grab onto" anything with spider climb, we can assume that the check to grab onto something to prevent the fall is passed automatically.
At the very least, when affected with the spell.

A DM might rule that a monster with spider climb still has to make a dexterity check to change into "climbing" mode, especially if it's a biped – like a vampire – that isn't naturally in climbing position.

Answer 2

The Reverse Gravity spell's description as quoted is very clear and explicit (emphasis mine):

All creatures and objects that aren’t somehow anchored to the ground in the area fall upward and reach the top of the area when you cast this spell. A creature can make a Dexterity saving throw to grab onto a fixed object it can reach, thus avoiding the fall.

The wording is very unambiguous: all creatures fall unless they are somehow anchored. The point of discussion is therefore: does having spider climbing somehow anchor you to the ground or not?

The exact wording may depend on your source of Spider climbing, but from what I can tell, the ability does not explicitly anchor you, instead it lets you climb those difficult surfaces without a check. For most sources I am aware of, activating it costs an action though, meaning there's a required element of 'intending to use it'. For example, when you're consciously walking towards a surface with the intent to climb it, you use an action to activate your spider climbing.

This leads me to conclude that the Reverse Gravity spell still affects creatures with spider climbing roughly equally: requiring a successful check of some kind if they want to remain in place. At their discretion the DM would have to decide whether this is still a Dexterity saving throw or some other check. Only once they've made their check successfully does spider climbing come into play, granting the climber their full walking speed.

This assumes that your source of spider climbing requires you to activate the spider climbing ability. If you inherently have this ability as a part of your character, then you could definitely argue that a check is not required, since when active the ability allows you to move freely on a surface, even if that surface is a wall or ceiling.

Answer 3

Reverse Gravity doesn't specify exactly how it interacts with every effect that could modify when and how a creature falls. "Spider climb" abilities are one such type of effect, but there are also things like flying, levitation, normal climbing, telekinesis, the restrained condition, among many others. The DM needs to make a ruling for all of these if they come up. For making these rulings, I recommend the following general principle: reverse gravity changes the direction in which things fall, but does not change the rules for how falling works unless otherwise noted. (The "otherwise noted" part would be the saving throw to grab onto something when the spell is cast, although even this is a common house rule for many DMs in the case of regular falls of cliffs and the like.)

Using this principle, a flying creature will be unaffected by reverse gravity, as will any creature capable of standing upside down on a ceiling. Thematically, they will certainly notice when they enter the area of the spell and have to make adjustments, but they will not fall and will not have any additional trouble moving. Likewise, a levitating creature, an object held by a wizard's mage hand, and a creature restrained by a spider web would not fall when the spell is cast. The dexterity saving throw to avoid falling by grabbing onto a solid object is not relevant for any of these creatures or objects, because they are already held in place by something other than gravity.

Of course this is not the only ruling you could make. You could take the spell's text literally and say that anything not anchored to the ground falls up to the top of the spell's area, regardless of what else may have been holding it in place. This includes flying creatures, levitating objects, and so on. It doesn't feel very satisfying to me that a mage hand holding a key in midair should be forced to drop it simply because the gravity it was already ignoring has reversed directions, but it's still a valid ruling given the spell's text. However, even with this ruling, I would argue that spider climb allows a creature to maintain their footing without a check or saving throw when reverse gravity is cast. I would say that within the spell's area, the floor has become the ceiling, and spider climb anchors the creature to the ceiling, preventing them from falling.