The Gimp is almost as good as Photoshop, so yes.
Personally, I like to:
(A) for organic rough grainy textures
1. Scale the texture times four using Nearest-Neighbor filtering
2. Apply a median filter (2 or 3 pixels, you choose)
3. Duplicate the layer
4. Use a splatter filter (in brush strokes for photoshop) on the second layer
5. Make said layer 50% transparent
6. Select splotches of color with Select Color Range
7. Add a tiny bit of noise to said splotches
8. Select different color ranges and repeat
9. etc etc etc, use your imagination
(B) for upgrading wood
1. Scale the texture times four using Nearest-Neighbor filtering
2. Apply a median filter
3. Create a new layer with harsh black & white noise
4. Apply motion blur in the direction of the wood (use your judjement for the length)
5. Use liquify/smudge/distorting filters on it, or not
6. Unsharp Mask the noise (harshly) to make lines stand out and contrasted
7. Set layer as overlay or soft light
8. Adjust brightness/saturation on this layer so that overall intensity of texture true to the original (squint at it)
9. Do the color range noise thing again, or not
(C) for upgrading drawings, details, etc
1. Scale the texture times four using Nearest-Neighbor filtering
2. Apply a median filter
3. Use smudge tool (1-3-5 pixels, depending, you choose)
4. Draw away on top of the scaled-up texture, in the direction of the details
5. Apply sharpen if you want
You can combine any and all of these techniques.
