DIAGNOSIS: That's not normal. The stock stand is generally sturdy. The wobble usually means the stand isn't fully locked into the monitor body, or your desk surface isn't perfectly level or is itself flexible.
FIX: Here's what to check. First, grasp the monitor head with both hands and gently try to tilt it forward and back. If it moves independently of the stand, the quick-release isn't engaged. 1) Carefully lift the monitor off the stand. 2) Look at the red locking lever on the back of the stand's neck—make sure it's fully pushed down into the 'locked' position. 3) Re-seat the monitor onto the stand until you hear a solid click.
IF THAT DIDN'T WORK: The issue is probably your desk. Try placing the monitor in a different spot on the desk. If the wobble stops, your original spot is uneven or the desk is too thin/light. Consider a monitor arm—the 27GP850-B has a standard 100x100mm VESA mount, and a solid arm will eliminate desk-born wobble completely.
HEADS UP: When you re-seat the monitor, do it on a soft surface like a carpet or towel in case you drop it. Don't force it; it should click into place smoothly.