GFM inverters have been widely recognized for their enhanced stability in weak grid conditions compared to GFL inverters. However, their power output capability (P–Q capability) may still be limited by both physical and stability-related constraints, particularly under low short-circuit ratio (SCR) conditions. While numerous studies have investigated P–Q capability in voltage control mode with constant voltage set points, a comprehensive characterization of P–Q capability in the widely adopted reactive power control mode, where significant voltage variations arise from elevated grid impedance, remains limited. This study provides a thorough evaluation of the P-Q capability in reactive power control mode, taking into account the combined constraints of inverter voltage limit, current limit, angle stability limit, and voltage stability limit. The explicit mathematical models of these capabilities are developed in a unified <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P-Q</i> framework, incorporating the effects of the interface filter and virtual impedance. Our analysis demonstrates that the angle stability and voltage stability align along the same capability curve in reactive power control mode, suggesting that both challenges emerge concurrently. Leveraging these models, two adaptive reactive power control strategies are proposed to maximize the active power output while optimizing the reactive power and output current, respectively. The effectiveness of the proposed models and control strategies is substantiated through experimental results.