A demand response scheme that uses direct device control to actively exploit prosumer flexibility has been identified as a key remedy to meet the challenge of increased renewable energy sources integration. Although a number of direct control-based demand response solutions exist and have been successfully deployed and demonstrated in the real world, they are typically designed for, and are effective only at small scale and/or target specific types of loads, leading to relatively high cost-of-entry. This prohibits deploying scalable solutions. The H2020 GOFLEX project has addressed this issue and developed a scalable, general, and replicable so-called GOFLEX system, which offers a market-driven approach to solve congestion problems in distribution grids based on aggregated individual flexibilities from a wide range of prosumers, both small (incl. electric vehicles, heat-pumps, boilers, freezers, fridges) and large (incl. factories, water pumping stations, etc.). By encompassing individual prosumers, aggregators, distribution system operators, and energy multi-utilities and retailers. It is a system of systems, where all flexibilities in electricity demand, production, and storage are extracted, (dis)aggregated, optimized, and traded using the powerful and standardized FlexOffer format, yielding a general and replicable solution with low cost-of-entry. The system has been successfully deployed in Switzerland, Germany, and Cyprus where it has controlled loads of 500+ prosumers, with a total of 800MWh flexibility offered on the market, offering up to 64% of adaptability in peak demand. In this paper, we present the overall architecture of the GOFLEX system, its sub-systems, and the interaction between these sub-systems. We then discuss the configurations, observations, and key results of using the GOFLEX system both in the aforementioned 3 demo sites-within the GOFLEX project and after the project.