I know which file it would be that stores the data for the outfit, and it is there in the GOG version (ch_m_chloe_deluxe05.bytes) but unfortunately I don't know which file unlocks the outfit in-game. That's a bummer. If I figure out which one it is I'm happy to share my steam file with anyone. Big IF tho.
On the bright side: the preorder bonus "Classic Chloe" (ch_m_chloe_deluxe04.bytes) is part of the GOG version. That's awesome, right?
EDIT1: To clarify why I'm even doing this: Linking your account is not possible on GOG, for two reasons. BtS doesn't even have the option in the main menu, and although LiS does have the option, there is no online service, so it still doesn't work.
EDIT2: No, sadly it's not possible (see EDIT1 for the reason). Nothing's written into any file, it is an online feature. You have to be connected to your Square Enix Account at any time if you want the outfit unlocked. Running Steam offline will have the outfit removed from the selection.
Post edited June 08, 2020 by GrizzledLone