Gibson's vet, Dr. Peter Walsh said:
Last week Gibson began having trouble using his back legs. X-rays showed that the cancer had spread to his spine and his lungs. Ms. Hall made the very difficult decision based on her concern and love for Gibson to have him humanely euthanized. Gibson died peacefully in the loving arms of Ms. Hall.
See a classic video of Gibson showing off his tallness and generally being cute when you read more.