You can create grunge images in GIMP many different ways. In fact, there is no right way to do this, and mixing techniques can make for even better effects. A lot of the process involves throwing things into the image and seeing what sticks. It might take a bit of practice to make them look really good. The goal is to create a small-ish image that we can tile on a webpage.
Before going further you'll want to find some grunge brushes to make this easier. These work great, and you can find others as well. It's hard to have too many grunge brushes.
Start off with a GIMP image and fill the entire layer with whatever base color you want. I'm using #e1d9c2.
Add a new layer and select white as the color. Start going to town with the brushes. Don't worry about how it looks, none of the detail will be retained. When you're finished, go to Filters > Blur > Motion Blur. Select 256 for length and 180 for angle. Then do it again with 256 length and 360 angle. This will give create horizontal streaks that give a decent base. You can play with the angles if you don't want horizontal. In the layer dialogue, change the layer mode to Overlay.
Add a new layer and select black as the color. Repeat the steps for the last layer. You should have two layers with streaks at the same angle.
These layers are responsible for the texture feel. Thankfully we can automatically generate the effects for this layer. Additionally, depending on what you're doing, this step may be enough to suit your needs.
Create a new layer. There are several ways to do this. First, try Filters > Render > Clouds > Plasma. I like to bump the turbulence up as far as possible but you can try any value. Regenerate the random seed until you're happy with the preview. Chaos is good here. This will create a really colorful and weird looking image. Go to Colors > Desaturate and you can choose whichever option you prefer. I usually do Lightness. Afterward, change the layer mode to Overlay and mess with the opacity if you need to.
Repeat the same process for Filters > Render > Clouds > Solid Noise. Once again I bump up the X and Y size all the way, with a high amount of detail, 10-15. Hit ok. Change the layer mode to Overlay, adjust opacity, etc.
Now we're going to add some actual detail. Add a new layer and use the grunge brushes to make some cool effects. Play this by ear. Use white brushes on the layer. Change the layer mode to Overlay. Repeat this process with a black layer. Play with the opacity settings to make each layer look good.
Add as many of these layers as you want.
If you only want to have a single image somewhere, don't worry about this step. This step is important for webpages so that you can tile the image to make it cover an entire background. If you tile an image without modifying it any, it will usually be very obvious where the edges of the image are. We want to make a tiled background that looks like one big image. It would be possible to create a really large image so you didn't have to tile it, but then each client has to download a large file, which is unnecessary and might as well be avoided.
Besides, it's really easy in GIMP. Simply go to Filters > Map > Make Seamless. Voila. Now you can tile the image and it'll look much better.
Alternatively, if you're not tiling, you may want to rough up the edges of the image for a cool effect.
You can apply these concepts to create grunge effects in other ways. For instance, when creating grunge text, these layer effects will be applied on the text only. And you probably won't need to do every kind of step here. You may not want a motion blur background, or you may only want the subtle texture without the added grunge detail. Just mix and match for your own needs. Enjoy!