So here are the differences between Eiyuu Senki and GOLD:
GOLD isn't a SEQUEL, it is A SIDE STORY! AN ALT STORY IF YOU WANT!
The MC: In the original, he's an Isekai'd MC! In GOLD, he already is in the Eiyuu Senki world (part of it)
Sex Scenes: GOLD has more sex scenes than the regular one even though you can't still do it with everyone. Rejoice still. GOLD however, has a special treat for you: If you defeat a girl with a special attack, her clothes will tear off!
Story: This is the lackadaisical part about GOLD since the story isn't as good as the original. I played the game in Japanese so I didn't understand a lick about it. I have to play this one to differentiate but it is still a lackluster story.
Gameplay: In contrast, gameplay has been drastically improved! The Mages for example are given a better purpose, some characters got rebalanced and some got nerfed to the ground. Some also got ALT Personas. Also, there is Item Fusion in order to collect more items!
Favorite characters (design-wise): ALEXANDER (Full Heterochromia and literally the only waifu that WEARS A WEDDING DRESS in this game).