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Modern extragalactic molecular gas surveys now reach the scales ofstar-forming giant molecular clouds (GMCs, 20-50 pc). Systematic variations inGMC properties with galaxy environment imply that clouds are not universallyself-gravitating objects, decoupled from their surroundings. Here we reexaminethe coupling of clouds to their environment and develop a model for 3D gasmotions generated by forces arising with the galaxy gravitational potentialdefined by the background disk of stars and dark matter. We show that thesemotions can resemble or even exceed the motions needed to support gas againstits own self-gravity throughout typical galaxy disks. The importance of thegalactic potential in spiral arms and galaxy centers suggests that the responseto self-gravity does not always dominate the motions of gas at GMC scales, withimplications for observed gas kinematics, virial equilibrium and cloudmorphology. We describe how a uniform treatment of gas motions in the plane andin the vertical direction synthesizes the two main mechanisms proposed toregulate star formation: vertical pressure equilibrium and shear/Coriolisforces as parameterized by Toomre Q~1. As the modeled motions are coherent andcontinually driven by the external potential, they represent support for thegas that is distinct from that conventionally attributed to turbulence, whichdecays rapidly and requires thus maintenance, e.g. via feedback from starformation. Thus our model suggests the galaxy itself can impose an importantlimit to star formation, as we explore in a second paper in this series.

A Model for the Onset of Self-gravitation and Star Formation in Molecular Gas Governed by Galactic Forces: I. Cloud-scale Gas Motions