Ancient-Red-Dragon: The GOG version of Yakuza 0 100% for sure does indeed have the bug where RC parts can become placed in your equippable equipment slots, and then they become stuck there permanently.

That happened to me with the GOG version of Yakuza 0.

I was able somewhat to un-screw up my inventory by making sure all of the other equippable gear slots were filled, and then acquiring an additional gear item on top of that.

Doing this automatically sent the bugged RC item into my storage box, and therefore it once again freed up my previously-bugged equippable gear slot; in other words, it un-bugged my inventory, in exchange for somewhat-bugging my storage box.

But if I ever try to remove that bugged RC item from the storage box, then the bug will reoccur, and it will fill up all of the free gear slots with that same one bugged item, for as many times as I try to withdraw it into a free inventory slot.

You could always try that same method as well, in order to be able to resume playing with your existing saved games from the Steam version.

If you get that bugged item into your storage box by way of filling up all of the remaining inventory squares, and then acquire a new item of the same type (for me, the RC part went into one of my Accessory inventory squares, so I needed to acquire a new Accessory item in order to un-bug that slot, if I remember correctly), and then simply never again withdraw the bugged item once it's in your storage box, then your inventory slots should become restored to normal functionality.

And then you won't have to worry about this bug problem affecting your gameplay anymore (other than you have to remember never to withdraw the bugged item from storage, and also you have to scroll past the bugged item forever whenever you browse through that type of item in your storage box...but those are very small issues as compared to the original problem which is much worse).

Definitely they never bothered to remove this bug from the GOG version, unfortunately; it's still there, same as with the Steam version.