Edit gif

[Krikor]

An idea:
Create a new transparent layer.
Bearing in mind which parts you want to remove, paint with the brush on this transparent layer over the equivalent areas of the other layers that you want to remove.
Then use the ofn-interleave-layers.py plugin

The painted transparent layer will be overlaid on top of all the layers of the gif (and merged to each one)

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Watching the rich2005 video from the previous post I realized that using Filters-Animation-Overlay background makes the idea I proposed even simpler.
The video also served to remind me of other useful plugins that I didn't even remember anymore.
Thank you rich2005!