Gap metric and stability margin have been proven as important model-based analysis tools for linear time-invariant (LTI) systems. Due to the need for an accurate model, both the gap metric and the optimal stability margin are mostly used for offline analysis. In online analysis, accurate models are rarely available, whereas measurement data could be easily obtained from the systems under consideration. Therefore, in this paper, an approach toward the calculation of the gap metric and the optimal stability margin based on the available measurement data of LTI systems is proposed. A data-driven realization of the so-called stable kernel and stable image representation serves as the foundation of this framework.