As both clouds follow a similar trajectory around the black hole, small changes in the objects' gas could be measured. And the evolution of these clouds revealed characteristics of the interstellar material surrounding Sgr A*.
"Although it is not yet clear whether these objects contain embedded stars, their extended gaseous envelopes evolve independently as gas clouds," they write. "We find evolution consistent with the G-clouds (G1 and G2) originating in the clockwise disc. Our analysis enables the first unique determination of the rotation axis of the accretion flow: we localize the rotation axis to within 20 degrees, finding an orientation consistent with the parsec-scale jet identified in X-ray observations and with the circumnuclear disc, a massive torus of molecular gas (approximately) 1.5 parsecs (5 light-years) from Sgr A*."
ANALYSIS: Event Horizon Telescope Will Probe Spacetime's Mysteries
Basically, observations of G1 and G2 show the direction that material travels as it falls into the black hole, thereby tracing out the rotation of the black hole's accretion disk. Also, they found that rather than the black hole being fed by the stellar winds of nearby stars, material is being pulled from a massive ring of material, some 5 light-years away.