Layer assignment is critical in global routing because different metal layers have different resistance and capacitance, which directly affects congestion, timing, and power. This paper proposes GAP-LA, a GPU-accelerated performance-driven layer assignment framework for holistic optimization of these objectives. The method achieves improved WNS and TNS with competitive runtime on large-scale designs, and further improves timing when applied to topologies from ISPD 2025 contest-winning flows.