log in as tdq_admin, then go in "analytical model > Document template". Browse the tree down to "Functionalities > tdq_user > dq_integrity_dashboard"
There, you'll find a document called "dashboard out of thresholds" which is the composite document that defines the layout of the gauges. Then, each gauge is a jrxml file that you can find in this same folder: for instance "Out of thresholds in percentage" You may edit these kind of files with the iReport studio from Jasper.
Have a look at the DQ Portal adminitration guide for a quick overview of the DQ Portal administration and development. You may also want to visit http://wiki.spagobi.org/xwiki/bin/view/Main/ for a more complete information about SpagoBI.