My understanding of the use of gibbets was that they were cages for the bodies of executed felons. Bodies were left to decay and were often eaten by birds; some were covered in tar to prolong their existence, which is one possible origin of the term black man/boy. They were placed at the site of the crime as a warning to deter miscreants and to remind locals that crime did not pay. But at least in some places, it seems the use was more gruesome. This is from Derbyshrie, near Chatsworth House.
Long ago, so the story goes, while a woman was frying some bacon in her cottage by the moor side a tramp came to the door. He asked for food; she said she had none to give him. He pointed to the frying pan; the woman retorted that she was not cooking bacon for idle folks like him. Thereupon the ruffian attacked her, knocked her down, and in blind fury poured the boiling grease from the pan down her throat, scalding her to death. The miscreant was sentenced to be hanged alive in chains by the cottage door where his victim had lived, and there the gibbet was set up. Her was long in dying – it is said that a passing traveller took mercy on him and gave him food – and his creams, as he swung on his gibbet, were so piecing that they disturbed the peace of the lord of Chatsworth in his house over the hill. Thenceforward, the legend adds, no criminal was ever gibbeted alive in Derbyshire.