Analyses of gamma-ray line emission in solar flares have provided information about conditions in flaring magnetic loops, the abundances of the chromosphere where the gamma rays are produced, and the composition and spectrum of the flare-accelerated ions. While laboratory measurements of the cross sections for production of the strongest lines seen in flare spectra are available, these measurements often only cover a limited range of projectile energies. In addition, the bulk of the gamma-ray emission arises from the numerous weaker lines for which there are no measurements. The gamma-ray de-excitation-line production code, developed originally by Ramaty, Kozlovsky, and Lingenfelter, has been and continues to be the primary theoretical tool used for analyses of solar-flare gamma-ray data. The code uses both measured cross sections and estimated cross sections where measurements are inadequate. We have improved the completeness and accuracy of this code in three ways. (1) We use recent cross section measurements to improve cross sections for those lines already explicitly included in the code and to provide cross sections for new explicit lines. (2) For the first time, we give a detailed evaluation of the unresolved-line "continuum" (i.e., all line emission not accounted for by the explicit lines in the code). Because adequate laboratory measurements for this emission are not available, the primary tool for this evaluation was the theoretical nuclear program TALYS. We explore how this unresolved-line continuum depends on parameters relevant for solar flares. (3) We use TALYS to improve those line cross sections where available laboratory measurements are inadequate and to provide cross sections for new explicit lines for which no measurements exist. Numerical cross section values for all lines explicitly addressed by the code and for the unresolved-line continua are given in the Appendix.