Gabbeh rugs are Persian, true Gabbeh will come from NW Iran region.
Its important to know this because many are copied and manufactured for western sales which will not be a true Gabbeh.
As mentioned they can be irregular in shape although not all, they have a thick pile with very simple designs.
Cleaning should not pose any real problems if carried out in the correct fashion.
What I would strongley suggest is that you take your digital camera along and take several photos insitu before cleaning/removing from site.
It's amazing how many clients will think that you have altered the shape of the design further by cleaning you can quickly prove you haven't with your pictures
Good Luck.