Hello, I wonder if there is any way to add Music Discs from other mods to the disc-gui

I wanted to update my modpack to 1.21.1 anyways so I'll just wait for everything to update

Prior to 1.20.6 excluded you need a resource pack to add custom music disc to the gui. After 1.20.6 included, they are automatically added when you enter a world with custom/modded music discs

I use 1.20.1 for my modpack so which resourcepack is it?

You have to make it yourself, there is documentation here and you can find example here