Say a mage with Spirit 4 creates a gateway between the Umbra and the Material World.
What happens when a spirit in the Umbra crosses that gateway into the Material World?
A relevant quote about Stepping Sideways from M20's main book, page 475:
To start with, a mage who steps sideways must use a Spirit 3 Effect to step sideways, employ Spirit 4 to open a gateway in the Gauntlet, or travel with someone else who opens that gateway for him. Certain powerful spirits can bring a mortal over to their side as well. Whichever way, the traveler becomes ephemera: the material of the spirit worlds. He might feel like flesh and blood, but he has become like the spirits around him.
In game terms, there’s no change between matter and ephemera. The character still uses all of his Traits the same way on both sides of the Gauntlet. Once he’s in the spirit world, however, that traveler cannot affect the physical world without using Spirit Sphere magick to reach the other side. He may, however, see and address spirits as if they were flesh and bone – something he can’t do when he’s in the material world. Even so, he needs at least two dots in the Spirit Sphere before he can actually touch them, and Life and Matter magick have no effect on spirits at all. (For details, see Part VIII: Umbral Spirit Entities.)
When the traveler crosses back to the mortal world again, the process works in reverse; he becomes material flesh again. On both sides of that crossing, the experience can feel pretty weird – after all, the traveler’s exchanging one kind of form for another.
Emphasis mine. It's clear that material beings get transformed into ephemera when crossing the gateway into the umbra, and they get transformed back into their material form when crossing back from the umbra into the material world.
What isn't clear is if this also works for ephemeral beings, do spirits materialize when crossing the gateway into the material world, as if using the Materialize Spirit Charm, even if they don't posses it?
If they do not materialize, what happens to them?
Wether the spirit is willing to cross the gateway or not is irrelevant, he could be forced to by someone else for example.
If the answer cannot be found in a Mage book, I would be interested in knowing if this is explained or mentioned in any of the other OWoD books.