This paper presents a GenAI-based automatic test case generation method that leverages Large-Scale Language Models (LLMs) and Vision-Language Models (VLMs) to transform natural language requirements and system diagrams into structured Gherkin test cases. By integrating automotive signal specification modeling, it standardizes vehicle signal definitions, improves interoperability between automotive subsystems, and simplifies integration with third-party test tools. The generated test cases are executed on the digital.auto platform, an open, vendor-independent environment designed for rapid verification of software-defined vehicle functions. We evaluate the approach using a child presence detection system use case, demonstrating a significant reduction in manual test specification effort and rapid execution of the generated tests.