Is switching to an AMD GPU worthwhile with a hardware G-SYNC monitor?

The answer depends on the exact monitor. Newer displays marketed as G-SYNC Compatible usually also support FreeSync, but the PG279QE uses a dedicated G-SYNC module and does not appear to support AMD's variable refresh technology. With an AMD card, you would likely lose hardware VRR, though the GPU itself would still work normally.

If smooth variable refresh is especially important to you, keeping an NVIDIA card is the safer choice with this particular monitor. If the AMD card offers a much better price or performance, switching is still reasonable—you just need to decide whether losing G-SYNC is worth the savings. Checking your typical frame rates and whether you notice tearing will matter more than the label on the technology.

I made a similar switch from an RTX 2080 Super to a Radeon 6800 XT. My display switched from G-SYNC to FreeSync and the experience was effectively the same because the monitor supported both standards. That probably will not apply to the PG279QE, but it shows that G-SYNC itself is not automatically a reason to avoid AMD.

People have also used AMD cards with non-FreeSync G-SYNC monitors without major problems. You can cap the frame rate slightly below the monitor's refresh rate to reduce tearing and keep input latency reasonable. It is not identical to variable refresh, but at a steady 1440p frame rate many users may not notice a big difference.