Don't want to write this by hand? GodCraft sets minecraft:movement.glide for you with a switch.Get GodCraftOverview
minecraft:movement.glide is an entity component: it goes in the components object of a custom mob, or in one of its component groups. A gliding flight move style, used by the phantom, where the mob swoops forward instead of hovering in place.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:movement.glide": {}
}