The main area that your character will be walking around in is made of worn concrete. This is brought on to site in huge slabs and laid down. The idea we're trying to give here is of a worn, old, and weathered surface:
Go to CGTextures.com and find ConcreteFloors0060 using the search feature. Download the small versions of image 02 and 05.
Open image 02 in GIMP. Select the Crop tool and use it to draw a box around the edges of six concrete squares as you can see here:
When you've fine-tuned the corners so that they sit in the middle of the steel edges (as shown in the screenshot), press Enter to complete the crop.
Now go to Image ¦ Canvas Size and de-select the chain icon.
Change Pixels to Percent. Now, enter 200% in the height box.
Click on Resize. You now have six concrete slabs at the top of the image, and room for six more as shown next:
Go to File ¦ Open as Layers. Select the other texture you downloaded. This now opens in its own layer.
Select...