The fabrication or exaggeration of an anecdote/scenario, or just general bullshitting from an old relative, usually Nan/Grandmother.
Can be rarely used as a derivative of "Gassing" in a general context w/out an OAP involved.
Nan: "Stop leaving that television on standby, you're charging me in excess of £50 a day on electricity bills!"
John: "Oh, behave it's like 14 pence a day. Stop Nan-Gassing."
Granddad: "Have you noticed the evil glare in our neighbour Sally's eyes whenever you make eye contact? She must be a serial killer."
Jason: "No, that's a genuine Nan-Gass, Granddad."