How to apply Local Contrast using GIMP?

Unsharp mask: blur the image, subtract from the original (to estimate the effect of blurring that we want to undo), add the result to the original.

Local contrast = large sharpening radius (to compare pixels to the local average, so the surroundings, not just the neighbouring pixels), low amount.

A great summary:

https://www.cambridgeincolour.com/tutorials/local-contrast-enhancement.htm