Geuze is one of the most complex beers in the world. To understand what geuze is, there must first be given an explanation about what Lambic is...
Lambic (Ambrose), a drink which was already mentioned in 1559, is a malt-grain mixture that at least 30% unmalted wheat and undergoes a spontaneous fermentation, ie: no yeast added to the brew. Fermentation is initiated spontaneously by the airborne "wild" yeasts. This spontaneous fermentation of Lambic is unique to Belgium, and can only happen in Pajottenland and the Senne Valley, south-west of Brussels. The wild yeast, Brettanomyces species including the famous, fertilize the 'wort' and afterwards the lambic is stored in large wooden barrels. In these large barrels the beer is aged for several years. Lambic is a flat, foam free beer. The flavor of lambic varies with the age of the Lambic. Young lambic initially tastes sweet and is evolving into something sweet and sour. Old is soft to sharp acidic tasting.
The changing zeitgeist, consumers wanted a beer with foam and some insightful Lambic brewers accomplished this and give birth to Geuze. The lambic brewers fit the champagne method to create a new beer. It was a blend of Lambic of different vintages and pulled it in bottles. Gueuze is still true today by old (fermented) to mix with young Lambic Lambic. The young lambic provides the nutrients (sugars) for a new fermentation. Through this new fermentation, which occurs in the bottle, as coveted yielded the foam collar. Real geuze is sold in champagne bottles that can handle the pressure of fermentation, other bottles would just break.
Old Gueuze is made of 100% Lambic and not sweetened. Girardin (black label), Cantillon, Three Fountains, Boon Oude Geuze and Old Belle Vue Selection Lambic belong here. (PS: the common Belle Vue Gueuze is not a traditional but commercial beer that is hardly worth the name geuze). The following beers can probably also be listed here but are a bit special: Eylenbosch Geuze Vintage 1984 (Limited Edition, closed in 1991) => if you come across this and want to open it for tasting contact me!, Lindemans Oude Geuze Cuvée René, Hanssens Oude Gueuze, Moriau Oude Geuze, Boon Oude Geuze Mariage Parfait.
Traditionally made geuze but sweetened. The breweries that do this are Mort Subite, Timmermans, Lindemans, De Troch, Girardin (White label) ... Sometimes they have a traditional batch in their range.
This geuze is based on high fermentation beer, mingled with beer of spontaneous fermentation and pasteurization. Belle Vue Gueuze is an example of this category (personal note: this is tasting like water used to do the dishes).
Some other beers of spontaneous fermentation exist but cannot be called Geuze / Lambic for example St Louis Fond Tradition and Jacobins Gueuze (Bockor). Variations of modified Geuze Lambic are fruit beers like cherry and framboise. But also Faro beer wich has an addition of dark crystalline sugar. And for the real fans pure unfiltered Lambik is also an option (if you do not mind the acidic taste) the best example is possibly Cantillon Lambik Grand Cru Bruocsella but also some of the 'Ancient' traditionele geuzes come very close in taste.