Gusset plates are used to connect diagonal members to vertical and horizontal members in steel structures. Notional loads are now the preferred method in the U.S. to consider geometric imperfections in steel frames; however, the current design method for gusset plate compression strength uses an equivalent column model with empirical effective length factors. A more versatile design method for gusset plate stability, which was developed in 2016 by the author, has been refined using existing experimental data and expanded to include other connection geometries. The notional load yield line (NLYL) method includes the application of an out-of-plane notional load near the end of the diagonal bracing member. The out-of-plane flexural strength of the gusset plate is calculated with the yield line method, providing a realistic strength-based model that is more accurate than the current semi-empirical approach. The NLYL method includes a calibration of the notional load magnitude with existing gusset plate tests, and the calculation of second-order effects using amplified first-order moments. The discussion will include the development of the NLYL method and comparisons with experimental data, showing the accuracy for various connection geometries.