In the process of exploring GIMP, trying to create new background textures for some future works, I came up with the following steps that usually give satisfying results (to me!):

1. first of all, create a new image with a solid colored background;

2. add some noise, using any of the following Filters:
-> Noise -> Casual (set a 5-6%)
-> HSV Noise (Holdness=2; Hue=3; Saturation=20; Value=20);

3. apply Filters -> Artistic -> Apply canvas (depth 3-5);

4. at this point, you can add some other Artistic filters, such as Oilify (mask dim.<8) or GIMPressionist (Paper=defaultpaper;  Scale=30;  Relief=6);

5. now create a new level;

6. choose a non-solid brush, set its opacity at 50% and start drawing casually on the new layer using a white color;


7. repeat pt. 5 and 6, this time using a black color;

8. now set these two last levels mode to overlay with an opacity of 50-60%.

There you go. Play with the settings of each filter, and you’ll probably come up with an interesting texture on which you can start creating your content.