Does Gravity Bow stack with Enlarge?
As in I cast enlarge on myself, as a medium creatue, and the turn after I cast Gravity bow and on the third turn I shoot with my longbow. Does it do Huge dmg instead of large?
They don't stack, but possibly not for the reason you are expecting. Gravity Bow can only be used on a missile weapon. An Enlarged creature wielding a missile weapon does not actually deal any extra damage, as the missile reverts to normal size after firing. See text below -
If you really want an example that does stack, though, you could try Lead Blades and Enlarge Person. Lead Blades only works on melee weapons, though. And to go for gold, add in a dash of Titan Mauler - this would allow you to be wielding Large weapons in the first place, and so have them deal damage as if they were Gargantuan :)
In D&D the question whether missle sizes existed at all in RAW was controversial. In pathfinder it's all still quite complicated as seen in this forum discussion, where it's argued that because reduce person actually reduces the damage (for projectile but not thrown weapons), the size of the launcher matters. For enlarge the wording is differently though!
Reduce: "Any reduced item that leaves the reduced creature's possession (including a projectile or thrown weapon) instantly returns to its normal size. This means that thrown weapons deal their normal damage (projectiles deal damage based on the size of the weapon that fired them)."
Enlarge: "Any enlarged item that leaves an enlarged creature's possession (including a projectile or thrown weapon) instantly returns to its normal size. This means that thrown and projectile weapons deal their normal damage."
This is contradicting. However the gravity bow spell affects the arrow, not the launcher, thus if you assume that the weapon that fires the projectile matters for determining damage, then the damage should stack with a spell affecting the arrow additionally. If you feel like somehow only the size of the arrow matters, then you're out of luck. Well almost.
Carry large arrows around, drop them before you enlarge yourself, then pick it back up. Now you can use your magically large bow and mundanely large arrows with the gravity bow spell. - Or hire someone to do that for you, and waste their actions attaching the bigger quiver after the transformation. Maybe have a handed familiar do that?
It says "All equipment worn or carried by a creature is similarly [reduced or enlarged] by the spell." - Not exactly sure if that means if you pick up the large arrows they'd instantly become huge (thus giving you a -2 to hit penalty) and then shrink down to large again once you fire them... or if the size adjustment for equipment only happens when first affected by the spell for the duration of the spell, and anything you pick up after that stays small... Good question actually.