Give some details of this image, size in pixels, RGB or Indexed, photograph or a drawing ... anything that might be relevent.
However,
Gimp is a raster (bitmap) editor. An image is made up from those pixels. Zoom in far enough and you see the pixels. That is not the way images are normally viewed. Some types of image, typically pixel art will always look 'blocky'
Depending on the image there are smoothing filters, the gmic plugin has several. Effectively you blur the image. The cure might be worse than the original.
edit: An example: https://i.imgur.com/kAG8G2g.mp4 Small, noisy photo, smoothing using the gmic Anisotropic smoothing filter.
However,
Gimp is a raster (bitmap) editor. An image is made up from those pixels. Zoom in far enough and you see the pixels. That is not the way images are normally viewed. Some types of image, typically pixel art will always look 'blocky'
Depending on the image there are smoothing filters, the gmic plugin has several. Effectively you blur the image. The cure might be worse than the original.
edit: An example: https://i.imgur.com/kAG8G2g.mp4 Small, noisy photo, smoothing using the gmic Anisotropic smoothing filter.