Drawing a simple rectangle layer?

[Krikor]

Other way:

Select the layer that has the desired dimensions and duplicate it.
You can duplicate it as many times as needed.
Then edit the new layers with the desired colors.

Another way using the ofn-layer-tiles.py plugin.

1- Create a new image (400x400 in this example);
2- Image - Guides - Grid
2.1 - Set the width of the small rectangles (100x100 in this example), Mode: Spacing of Guides (px), check Border guides;
3- Layer - Tiles - Split tiles (by width and height)...;
4- Provide the values for the width and height of the rectangles (100x100 in this example);

This will result in 16 layers of 100x100 (in the same image), each small rectangle can be edited separately.