Yesod is the foundation, the ninth sephirah, and it is the last stop before the material world. Everything descending the Tree of Life passes through it into Malkuth, which makes it less a place than a channel, and gives it the character of a reservoir rather than a source. Its planet is the Moon.
The Golden Dawn and the schools that followed it read Yesod as the astral, the storehouse of images and forms out of which physical events precipitate, and gave it dreams, visions, the imagination and the shaping power of habit. Zoharic Kabbalah reads the sphere as the point of the covenant and of the generative power, the narrow place through which the whole of the tree above pours into the world below. The two readings are not the same, but both describe a funnel.
Gabriel belongs here for a reason that fits the sphere exactly. His work in the older literature is not to originate anything: it is to appear, usually at night, usually in a vision, and to hand over a message whose author is elsewhere. Daniel receives him in the middle of a vision he cannot interpret. Joseph receives his instructions in dreams, and the tradition that names the angel of those dreams as Gabriel is old.
An angel of visions in the sphere of visions, and a lunar messenger in the sphere of the Moon. Of the ten sephirotic placements in this collection, this is among the few that needs no argument to defend.