The piston pressure gauge is a critical instrument for pressure traceability and measurement, with the piston system being its core component. Accurate positioning and rapid stabilization of the piston present significant challenges due to the properties of gas media. To address these challenges, a novel ultra-local mode-free adaptive control (ULMFAC) method is proposed, which analyzes the nonlinear characteristics of the piston system in gas piston pressure gauges. This method combines super-twisting nonsingular terminal sliding mode control (STNTSMC) with a finite-time disturbance observer, effectively preventing the chattering phenomenon typical of sliding mode control and greatly enhancing the system’s dynamic response. To account for parameter variations such as the effective piston area, temperature, and medium leakage, an improved second-order dynamics model is developed using an ultra-local model-free approach, eliminating the need for a precise system model, as required in model-based control methods. A nonsingular terminal sliding mode surface is designed to overcome singularity issues in terminal sliding mode control, while an adaptive supertwisting algorithm is applied to mitigate chattering and enhance system dynamics. A disturbance observer is used to estimate lumped uncertainties, ensuring finite-time stability. The stability and convergence of the proposed control scheme are confirmed through Lyapunov analysis. Simulations and experimental results demonstrate that the ULMFAC method significantly improves the robustness, piston positioning accuracy, and dynamic response speed under varying working conditions (0.5, 3 and 6 MPa). This method holds considerable theoretical importance and practical value for achieving high-precision and high-efficiency pressure measurements.