The debugging and validation of the many-core design is a complex task due to numerous events happening in the system simultaneously. Current state-of-the-art many-cores are strongly based on waveforms and log files to validate their behavior during simulation. Our hypothesis is that, as happened with ASIC development, a Graphical User Interface (GUI) can significantly accelerate the many-core development. To sustain that, we propose an open-source GUI tool called ManyGUI for many-core debugging. ManyGUI is organized in a framework that collects and classifies high-level events during simulation related to computation (executed CPU instructions), memory, and communication (NoC packets). Such events are shown graphically to the developer through a set of intuitive and practical frames. We evaluate ManyGUI in a silicon-proven state-of-the-art open-source manycore called OpenPiton, which uses RISC-V 64-bits CPU, 3 NoCs, and a distributed/shared cache memory organization. Results show that ManyGUI allows the developer to rapidly obtain a comprehensive view of the many-core behavior in terms of communication statistics (packets paths, link utilization), memory statistics (memory access, miss rate), and energy (CPU, memory, and NoC). CCS CONCEPTS • Hardware → Simulation and emulation; • Computer systems organization → Multicore architectures.