If you want to further justify the OYW-fielded Gouf Custom keeping up with more advanced machines, there's precedents for Zeon units being upgraded throughout the years with newer materials and technologies. Ironically one of those was Desert Rommel's own machine in ZZ, the Dwadge Reformation which could mount a humongous beam cannon and give the Zeta some trouble despite being essentially a souped up Dom. Then you have the Efreet Schneid (the purple one with a lot of knives) that turns up for a cameo in Gundam Unicorn, and Return of Johnny Ridden has Uma Lighting's custom Gelgoog which is souped up to match machines a decade more advanced.