In the cleared yard behind the jailhouse, the gallows is located; it looks "like a great L upside down."
Ma Tuck is locked up in the jailhouse in Chapter 24. She has been arrested for the murder of the man in the yellow suit. Ma Tuck had hit the man over the head, hard, with a shotgun, cracking his skull; the man had been threatening to take Winnie and make her drink from the magic spring so that he could use her as proof of the waters' unique powers to bestow eternal life. The man subsequently dies of his injuries, and Ma Tuck is arrested and taken to the jail to stand trial for murder. If she is convicted as seems likely, she will go to the gallows, and if that happens, it would be disastrous. Ma Tuck cannot die, and that fact would become evident to everyone at that point. The secret of the magic waters would be revealed, with potentially dire consequences.
The Tuck family and Winnie go to the jail in Chapter 24 to try to arrange Ma Tuck's escape. As they approach the jail, Winnie notes the threatening presence of the L-shaped gallows, standing ominously behind the jailhouse where Ma Tuck is being held.