The generic.gravity attribute cannot be applied on some entities such as arrows, items...

Steps to reproduce:

Place a command block (repeat, always) with the following command:

/attribute @e[type=minecraft:arrow,sort=nearest,limit=1] minecraft:generic.gravity base set 0.01

Try to shoot an arrow and nothing will happen (Except a message in the command block output indicating that arrows are not valid entities for this command).

Having this attribute working on all entities could be a lot better for servers that want to implement custom planets.