Flexible graphite-based gaskets are used extensively in high-temperature applications as a replacement for asbestos-based gaskets. The effects of aging and temperature exposure on flexible graphite sheet gaskets was the subject of a previous work [1,2]. In this paper, the effects of aging on flexible graphite under a confined-gasket configuration will be examined as they are yet unknown. This study outlines the performance evaluation of the elevated temperature behavior of flexible graphite-based gaskets under a confined configuration and long-term exposure employing a HALR (High-temperature Aged Leakage Relaxation) fixture. This ARLA-like fixture can retain the mechanical features of the ATRS/HATR while enabling cold leakage rate and weight loss measurements. Four distinct semi-metallic gaskets with different graphite-confined configurations, namely corrugated metal, spiral wound, kammprofile and double jacketed, were evaluated within a temperature range of 800 to 1200 °F (427 to 649 °C) for an exposure time of 2500 hours. Graphite weight loss, gasket thickness change, leakage and tightness parameters, and creep and relaxation measurements were taken at regular intervals for each gasket style. To better assess the aging process, these critical mechanical and leakage properties were scrutinized; the influence of the degradation process, related mainly to graphite oxidation, was emphasized.