Gründerzeit' Architecture is a minor decision for 
Austria-Hungary and 
Austria.
The Gründerzeit' Architecture is a type of architecture that appeared in the 1840s. These architectures meant German industrialization and progress. They also signify liberalism to the period.
All one needs is Iron Railroads and Cheap Iron researched to enact the decision. It also requires that Austria actually owns Vienna, but since it is the capital, it is very unlikely that Austria will exist, but not own Vienna. Vienna will get +5 life rating points, 20% more liberal Viennese population, and 10 prestige. Liberalism can only spell bad capitalist decisions in the beginning, use this decision when Austria have a stable industry.
Ingame description
The 'Gründerzeit', 'The Founding Era', refers to a period, stretching roughly from 1840 to the mid 1870s, of rapid economic modernization and industrialization of Germany and eastern Europe in general, and Vienna in particular. The era became know among other things for a its particular style of architecture, inspired by neo-gothic themes and imagery and the new building styles that industrialization had made possible, as well as for the rising tide of political liberalism that came with it. Vienna would, during the era Support these new trends, and watch Vienna blossom.