This paper investigates the effect of gain and phase margins (GPMs) on the delay-dependent stability analysis of the pitch control system (PCS) of large wind turbines (LWTs) with time delays. A frequency-domain based exact method that takes into account both GPMs is utilized to determine stability delay margins in terms of system and controller parameters. A gain-phase margin tester (GPMT) is introduced to the PCS to take into GPMs in delay margin computation. For a wide range of proportional-integral controller gains, time delay values at which the PCS is both stable and have desired stability margin measured by GPMs are computed. The accuracy of stability delay margins is verified by an independent algorithm, Quasi-Polynomial Mapping Based Rootfinder (QPmR) and time-domain simulations. The time-domain simulation studies also indicate that delay margins must be determined considering GPMs to have a better dynamic performance in term of fast damping of oscillations, less overshoot and settling time.