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GRAVITY is essentially VLTI hardware that together provide near infrared VLTI (optical interferometry) functionality. It was put into operation in 2016, improving upon earlier VLTI functionality, which had been based on VLTI's earlier beam combiners, VINCI (VLTI commissioning instrument), MIDI (mid-infrared interferometric instrument), and AMBER (astronomical multi-beam combiner). GRAVITY incorporated a new beam combiner (BCI for beam combining instrument), and continued to use the existing AO system, MACAO (multi-application curvature adaptive optics).
GRAVITY+ indicates a subsequent upgrade carried out on VLTI and GRAVITY to increase its sensitivity, which went into operation in stages over 2024-2025. GRAVITY+ replaced the AO system with GPAO (for GRAVITY+ adaptive optics, with laser guide stars for each of the four VLT telescopes), offered a mode where simultaneously viewing a nearby star assists in interpreting fringes, improved GRAVITY's spectrometry through new dispersers, and improved the optics and the handling of stray light.