Ultrathin and extra-large single-crystalline Au microflakes (Au MFs) have a huge potential in applications ranging from nanophotonics to catalysis. Yet, wet chemical synthesis approaches cannot access this size range due to the proportionality between growth time, thickness, and lateral size. Concurrently, complexity and small MF areas restrict the use of 2D template-based methods. In all cases, subsequent transfer to a substrate remains challenging. Here, we demonstrate a facile, gap-assisted synthesis method that enables on-substrate growth of ultrathin and extra-large Au MFs. In particular, using a 43 mu m gap-size between two glass substrates and leveraging the directed-growth effect of halide ions, we achieve a high yield (similar to 90%) of Au MFs on glass with lateral sizes as high as 0.25 mm and thicknesses as low as 10 nm. Interestingly, for up to 25 h growth time, we observe a time-independent, ultralow average thickness of just 21 nm. A parametric synthesis study and an in-depth material characterization provide mechanistic insights into this extreme 2D growth mode. Overall, our gap-assisted approach greatly enhances the halide effect and results in a record-high aspect ratio of similar to 10(4). It thus opens new opportunities for on-substrate anisotropic growth strategies that would benefit emerging optoelectronic and photoelectrochemical devices.