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The Gaia space mission is crafting revolutionary astrometric, photometric andspectroscopic catalogues that will allow us to map our Galaxy, but only if weknow the completeness of this Gaia-verse of catalogues: what stars does itcontain and what stars is it missing? We argue that the completeness is drivenby Gaia's spinning-and-precessing scanning law and will apply this principle tothe Gaia-verse over this series. We take a first step by identifying theperiods in time that did not contribute any measurements to Gaia DR2; thesegaps create ribbons of incompleteness across the sky that will bias any studythat ignores them, although some of these gaps may be filled in future datareleases. Our first approach was to use the variable star photometry toidentify the 94 gaps longer than 1% of a day. Our second approach was topredict the number of observations of every point on the sky, which incomparison to the reported number of detections revealed additional gaps in theastrometry and spectroscopy. Making these predictions required us to make themost precise, publicly-available determination of the Gaia scanning law. Usingthis scanning law, we further identified that most stars fainter than $G=22$ inDR2 have spurious magnitudes due to a miscalibration resulting from athunderstorm over Madrid. Our list of gaps and precision scanning law willallow astronomers to know when Gaia's eye was truly on their binary star,exoplanet or microlensing event during the time period of the second datarelease.

Completeness of the Gaia-verse I: when and where were Gaia's eyes on the sky during DR2?