Sharpen

 There are many ways to sharpen a photo.

The most popular one in Gimp is unsharp mask.

There is also High pass.

Another way is to duplicate your image and desaturate the duplicate. Then go to the colour menu, down  to tone mapping then to Retinex. Apply it using the default settings. Once you have done that set the layer mode to overlay. You may want to change the opacity of that layer (the duplicated one) to taste. This usually brings put more detail in some photos

In GMIC there are many under details. It really depends what you want to sharpen. too. I go for Sharpen [gradient] as I do a lot of smaller photos and it is kinder.