I am thinking of making a janky deck and the card Gravity Sphere seems perfect for the job I need done, I'm just not entirely sure how it works in the long run. I looked at the rulings for it and it says that creatures on the battlefield and entering the battlefield lose flying, and my uncertainty with how this works comes in when you involve the latest set, Ikora. The deck is based around the idea that you use Luminous Broodmoth to make all creatures you control immortal and abuse ETB and other effects. Would Gravity Sphere prevent these fliers or do the counters overrule it?
Also, can anyone provide examples of cards that would work the same or better than Gravity Sphere for a task like this?
Would Gravity Sphere prevent these fliers or do the counters overrule it?
It depends.
If a creature enters the battlefield with a flying counter on it while Gravity Sphere is on the battlefield, the creature will still have flying (C.R. 122.1b). This is because the counter generates an effect that gives the creature flying and "receives a timestamp as it's put on [that] object", including if the object enters the battlefield with that counter on it (C.R. 613.7c, 613.1f; see also C.R. 122.6). This timestamp is later than the timestamp of Gravity Sphere (and the counter and Gravity Sphere's effect don't depend on each other), so the counter takes precedence over Gravity Sphere's effect for that creature (C.R. 613.6, 613.7, 613.7a, 613.7c).
The flying counter is thus similar to Mystic Visionary's ability, which creates an effect that gives Mystic Visionary flying "as long as seven or more cards are in your graveyard" (C.R. 108.1).
However, if a creature that normally has flying enters the battlefield (without a flying counter) while Gravity Sphere is on the battlefield, the creature won't have flying. This is because here there is no effect on the creature itself that grants flying (as with Mystic Visionary, for example), and also because Gravity Sphere's effect comes from a static ability and thus "isn't 'locked in'" and "applies at any given moment to whatever its text indicates", namely "[a]ll creatures" (C.R. 611.3a, 604.1).
Also, can anyone provide examples of cards that would work the same or better than Gravity Sphere for a task like this?
That is not a rule question. But compare an effect that says "All creatures lose flying" with one that says "All creatures lose flying and can't have or gain flying" (but no card currently has exactly the latter text — the closest is Archetype of Imagination). The latter effect would keep effects, including counters, from giving a creature flying, even if they have a later timestamp (C.R. 113.11).
Also, can anyone provide examples of cards that would work the same or better than Gravity Sphere for a task like this?
It depends.
If a creature enters the battlefield with a flying counter on it while Gravity Sphere is on the battlefield, the creature will still have flying (C.R. 122.1b). This is because the counter generates an effect that gives the creature flying and "receives a timestamp as it's put on [that] object", including if the object enters the battlefield with that counter on it (C.R. 613.7c, 613.1f; see also C.R. 122.6). This timestamp is later than the timestamp of Gravity Sphere (and the counter and Gravity Sphere's effect don't depend on each other), so the counter takes precedence over Gravity Sphere's effect for that creature (C.R. 613.6, 613.7, 613.7a, 613.7c).
The flying counter is thus similar to Mystic Visionary's ability, which creates an effect that gives Mystic Visionary flying "as long as seven or more cards are in your graveyard" (C.R. 108.1).
However, if a creature that normally has flying enters the battlefield (without a flying counter) while Gravity Sphere is on the battlefield, the creature won't have flying. This is because here there is no effect on the creature itself that grants flying (as with Mystic Visionary, for example), and also because Gravity Sphere's effect comes from a static ability and thus "isn't 'locked in'" and "applies at any given moment to whatever its text indicates", namely "[a]ll creatures" (C.R. 611.3a, 604.1).
That is not a rule question. But compare an effect that says "All creatures lose flying" with one that says "All creatures lose flying and can't have or gain flying" (but no card currently has exactly the latter text — the closest is Archetype of Imagination). The latter effect would keep effects, including counters, from giving a creature flying, even if they have a later timestamp (C.R. 113.11).
EDIT (Aug. 2, 2021): Edited.