Do benign gallbladder polyps grow or just cancerous ones?

Yes, both. Gallbladder "polyps" may be true polypoid lesions or cholesterolosis, related to fats in the gallbladder wall. Both lesions can "grow". If significant polypoid disease is found on ultrasound, most surgeons recommend cholecystectomy, as there is a small, but real, chance of true polyps becoming adenocarcinoma over time.
Both can. Most are benign, not cancers. Rapid growth (seen on sono), size 1cm or greater, and certain other criteria may push to removal.