Gradients
A gradients is a progrssion from one color to another. You can use this as an additionnal light source to improve your photos.
Layers
Simply put, layers are images stacked on top of each other.
Layers are mostly used in combination with a reduced opacity so that elements of the images behind are shown by transparency.
When to use:
Layers can be useful to emulate particle effects or to blend different images.
Oftentimes you will also benefit from duplicating layers in Gimp so that you can apply different settings to them while preserving the original one.