Evidently there are no gallstones. Try reducing fat intake
I have not heard of any method that can be used to flush it out. There are prescription meds that can be tried that may be able to help dissolve the sludge, but not everyone is a candidate for that therapy.
I have heard there are 'natural products' that suggest you can flush the gallbladder, but if one were to be able to 'flush' the gallbladder, the crystals in the sludge would probably cause problems as they exit the cystic duct, the bile duct and the sphincter of Oddi. It's difficult to imagine this is possible without harm, because when this happens naturally in those with sludge it can, in some, cause intense pain and result in continuing problems.