How can I make a popcorn ceiling less noticeable in GIMP without it looking obviously blurred?

A straight blur usually looks fake on textured ceilings. A better approach is to reduce the ceiling’s contrast and slightly lift its brightness so the texture remains but becomes less distracting.

In GIMP, select the ceiling and feather the selection a little for a smoother transition. Then either:

• Use Curves: sample a point on the ceiling, add a reference point, and make the curve around that point less steep to lower local contrast.
• Or simply reduce contrast and increase brightness on the selected ceiling.

If you still need extra smoothing, a Median filter can work better than standard blur because it suppresses small texture while preserving broader tone. To keep it subtle, apply it gently and lower the layer opacity or use a lightening blend mode such as Screen.

For thin leaves or other small objects crossing into the ceiling, precise selection may not be worth the effort. If they are unimportant, use the Clone/Heal tool to paint them out or tidy the edge after the main adjustment.