Based on the modeling of the geometrically dependent gain coefficient (GDGC) and rate equations, this theoretical study explains the spectral linewidth behavior in an Se x-ray laser operating at 206.4 Å. The study used the reported experimental measurements for an Se target of 6.3 cm in length for the analysis. To obtain a finite value for the amplified spontaneous emission (ASE) intrinsic linewidth that initiates at the threshold length, we subtracted the ASE intensity background from the calculated ASE intensity to derive the corresponding FWHM. For homogeneously and Doppler broadened linewidths of 14 mÅ and 36 mÅ, we calculated the ASE Voigt profile linewidth to be 44 mÅ, initiated at the threshold length of 0.13 cm. The interaction is rather insensitive to the collision broadening; for the homogeneously broadened linewidth up to 21 mÅ, we could still explain the linewidth measurements. In this case, we calculated the related Voigt profile linewidth to be 48.5 mÅ. The current GDGC modeling approach greatly simplified the ASE analyses of these lasers.