This is my palette. It doesn't have to be neat and pretty.
On your palette's image click Filters>Colors>Smooth Palette. Your settings can be anything. I usually pick 300 pixles and 50. I try to work in numbers that go well with 3600 since that's the size of my papers for digital scrapbooking.
Take your new image, you can close the palette one if you wish, and stretch it to the size you need your plaid fill to be. In my case, I use 600x600, for this example I used 300x300.
Go to your layers tool box and copy your first layer.
Select New Layer, go to your image and click Layer>Transform>Rotate 90, as in the photo. Do not use Image>Transform or you will just rotate all layers at once.
Click on layers dialog and new layer, and play with the Opacity until you find what you like. Sometimes you might like 50, other times 60. Above I used 52.6. This part is about your taste.
When you are happy with the image, click Image>Merge Visible Layers.
You can do one of two things now. The first is to just save it as a jpg or png image then copy and paste into your patterns folder, go back to your layers tool box, click the patterns tab and refresh. Or you can click on the image Script Fu>Selection>To Pattern.
This is a sample of my finished product after bucket filling the pattern into a new image. The pattern remember was 300, this image is 600. If you are creating a webpage or scrapbooking background, you may want to stretch your stripes more to make them less busy. But remember stretching too much will blur your edges. It's actually a nice touch if you also add a texture like canvas, crackle, crumpled, etc.
You do not have to use smooth palette to get your striped look. You can use Filters>Distort>Blinds with layers of color, but you won't get the randomness shown above. You can create any kind of striped image by using selection tools, brushes and bucket fills as well.