This work presents a novel application of deformable mirror (DM)-enabled laser beam shaping to enhance gap bridging capability in autogenous butt joint welding of stainless steel sheets. Three distinct near-Gaussian beam geometries — circular (C), transverse elliptical (TE), and longitudinal elliptical (LE) – were experimentally evaluated for their influence on weld quality. The tests were conducted across constant joint gaps up to 0.6 mm in 2.0 mm-thick AISI 316 stainless steel. The investigation demonstrates that elliptical beam shapes, particularly those with the major axis aligned in the welding direction (LE), significantly improve gap-bridging performance and weld seam consistency. Metallographic analyses, in-process top-view weld pool imaging, and microhardness profiling collectively indicate that elliptical beam shapes reduce the sensitivity of the fusion zone geometry to gap variation, enabling bridging gaps up to 30% of sheet thickness, three times the typical 10% limit. The results further reveal that effective gap bridging is governed not solely by beam radius, but critically by the spatial distribution and gradients of power density across the gap, which modulate Marangoni-driven melt flow. These findings underscore the fundamental role of adaptive beam shaping in controlling weld pool dynamics and advancing process robustness without filler material. This study represents the first systematic exploration of DM-based elliptical beam shaping in autogenous butt welding and establishes a foundational methodology for real-time adaptive beam control in laser processing.