This links to Astronomy.com's Scientists finally confirm the Milky Way has a supermassive black hole
This links to ESO's eso1835 — Science Release Most Detailed Observations of Material Orbiting close to a Black Hole; ESO’s GRAVITY instrument confirms black hole status of the Milky Way centre
FYI, the GRAVITY instrument measures infrared light, not gravity.
What is it exactly about these flares of infrared light from Sgr A* that confirms it is a supermassive black hole?
Certainly there is plenty of evidence, the orbits of stars nearby for one. But this use of the word confirm seems strong and frequent enough to suggest the deal has been done, the ink is dry, and it's a black hole without question and that until now that wasn't so.
I've shamelessly absconded the following images from this answer to the question What is the evidence for a supermassive black hole at the center of Milky Way?, which may need a new answer!
Note, the size of these stellar orbits are compared to those of Sedna, Eris, Pluto and Neptune in the bottom right corner (same scale)!