Both are 27-inch monitors, each serving up a 2560x1440 (QHD) resolution on a curved panel, and both support HDR content. However, there are a couple of key differences.
The G-Sync monitor is built around a TN panel and offers a higher 165Hz refresh rate, while the FreeSync display uses a VA panel with a 144Hz refresh rate. it's impossible for us to know how good either one looks without seeing them in person, though generally speaking, IPS panels tend to look the best while TN displays favor speed over image quality. VA panels sit somewhere in between. So in terms of image quality, it's IPS > VA > TN. That's not a hard and fast rule, though.
Viewing angles (and by extension, the 'sweet spot') varies by panel type as well. In this case, the FreeSync display offers wider viewing angles at 178 degrees (horizontal and vertical), whereas the G-Sync monitor narrows things down to 170 degrees vertical and 160 degrees horizontal.
Pairing HDR support with variable refresh rate technologies (G-Sync and FreeSync) is what makes these monitors interesting, though neither one gets anywhere close to the 1,000 nits brightness level that really makes HDR content pop. Instead, they both peak at 400 nits. The trade off is cost.