The biggest update is support for recently released GPUs like the AMD Radeon VII, and Radeon HD 8400, as well as the Nvidia GTX 1660 Ti, Titan RTX, RTX 2080 Mobile, RTX 2070 Mobile, RTX 2060 Mobile, Quadro RTX 4000, GTX 650 (GK106), and the Quadro P5200. It also adds support for Intel's Amber Lake GT2 (Core i7-8500Y) and the support for detection of AMD's latest Radeon Adrenalin 2019 version drivers.
In addition to the new support, the new GPU-Z version got the simplified sensor titles like GPU Clock, Memory Clock, and Shader Clock, and unified Memory Used/Memory Usage sensor titles to Memory Used. It also brings improved crash reported, Advanced Panel, and more.
The biggest bug fixes were improved monitoring on Radeon RX 580 2048-SP, default boost frequency reporting on GTX 1660 Ti and certain "Pascal" GPUs, and missing fan sensors on RTX 20-series cards with no display connected.
Plenty of other bug fixes were included and you can check out the complete change-log over at the GPU-Z v2.17.0 download page.