天面 is a slightly unusual compound — not a common standalone word, but transparent from its parts: the "heaven-facing surface," i.e. whatever plane of a target points skyward. As a technique name it reads almost architecturally, like a term you'd expect in stonemasonry or rock-quarrying rather than swordplay, which fits Gyomei's whole naming register: Stone Breathing consistently favors this kind of geological/structural vocabulary (compare Form One's mineralogy term 蛇紋岩) over the mythic or meteorological language found in styles like Wind or Thunder Breathing.
VIZ's "Upper Smash" keeps the vertical direction ("upper") but drops the "heaven" imagery entirely, turning a phrase that gestures at something almost cosmological — striking the sky-facing plane of an enemy — into a flatly physical impact description. Given how the same translators keep Gyomei's Buddhist prayer chants and imagery intact elsewhere in the series (his constant nenbutsu recitation, his prayer-bead weapon), the loss here reads less like avoidance of religious content and more like ordinary compression: "Upper Smash" is punchier as a shouted attack name than any literal rendering of 天面 would be in English.