Gueuze is the result of a long process before arriving at the final product. It combines several techniques throughout manufacturing. One of these techniques is spontaneous fermentation.
You should already know that to obtain the final product, it is necessary to assemble several lambics which have different maturation times and even different origins.
Lambic is the basic beer needed to make gueuze. It is produced by brewing a wheat wort to which no yeast is added. Yeast is actually naturally present in the air and that is why it is called spontaneous fermentation.
For it to be present in sufficient quantity, certain climatic conditions must be met, such as precise temperatures and atmospheric pressure levels. This is why breweries producing lambic only do so at two times of the year: in March and September (note that this may vary depending on the climate).
Once the lambic has been brewed, it is left to rest in oak barrels for one to three years where it will evolve while taking on the taste of the barrel. It is only after this long process that two to three lambics are blended and bottled. Then comes a phase of rest in a hot room which will give sparkle to the gueuze.
The manufacturers of lambics are obliged to brew in the Senne valley, where the microclimate necessary for the presence of the famous yeast allowing spontaneous fermentation is found.