Otherwise referred to as the Thing in the Cellar, this is the Hex analogue, independently devised by Hubert Turvy to precisely replicate the circulation of money in Ankh-Morpork, and then with the aid of Igor, made absolutely perfect. Being constructed between a borderline mad scientist and an Igor, this can have some worrying implications. Hubert refers to the Glooper as "an analogy machine," but it is, in point of fact, a water-based analogue computer.
The city affects the Glooper, which shows not only the current economical situation in Ankh-Morpork, but can also show what may happen in theory, and more worryingly, what happens in the Glooper can affect the city, as seen at the end of Making Money. It has been reflected upon that taking a hammer and smashing the Glooper would result in a complete and total economic crash.
As TP himself points out in the Author's Note to Making Money, this is directly taken from Roundworld's Phillips Economics Computer, a bizarre but effective water-powered machine  devised in 1949, but which was so good at reproducing an economic cycle that the last of the Phillips machines were still going strong in economics schools in the early 1990s - when orthodox computer power had finally caught up with hydrology, and could at last replicate what the machines had been doing for nearly fifty years. The operators must have cheered this, as they could finally hang up the oilskins and sou'westers they had needed to wear indoors. The MONIAC - note one significant letter away from being a MANIAC - was also, with superb Discworldian ontology, called a Financephalograph.
(Incidentally, in case anyone might not have fully grokked the general computer engineering mindset, there was indeed a series of early computers called MANIAC, but these used conventional vacuum-tube technology.)
There is also an earlier reference to a device created by economist Irving Fisher around 1888 when he was writing his doctoral thesis. According to the book "The Myth of the Rational Market" by Justin Fox, Fisher "designed and built a contraption of interconnected water-filled cisterns that he described as 'the physical analogue of the ideal economic market'" (Chapter 1).
Interestingly, a protoype nuclear reactor, one of the first to be built in the world, was called the GLEEP by its British inventors. Terry Pratchett worked for the British nuclear power agency, a sucessor to the company that built the GLEEP, and cannot have been unaware of this. "Gleep" is also the name of a fictional substance (a foodstuff, I believe) in a humorous science fiction story of the mid-20th century which TP may also have encountered. It's a word which sticks with one, even though the story it appears in and the author who wrote it may well not.
Douglas R. Hofstadter, in his famous work Godel, Escher and Bach: an Eternal Golden Braid discussed the fictional and/or artificial programming languages BLOOP, FLOOP and GLOOP.