QWEEDDYZ: If you or Windows remove (or prohibited to read) entries for Bnet gateways game try to connect to old servers. Gog exe (all) detected as outdated.

If you try DX exe you get that message

If you manually start Updater (before try to go online), it overwrite Bnet Win REGEDIT entries as well as battlenet.sp file. And maybe even exe (although report it didnt do anything).

Anyway you need first reinstall the game even if at first it be temp offline server of Bnet. Now your version are downgraded. GOG Galaxy somehow possible to detect installed game as Galaxy build version, then verify installation (at least twice, first one redownload something). That also fix win registry.
Or look to info/script files (goggame_xxxxx) for correct registry. But still sp file most likely are downgraded.

Devuilition(X) authors say their mod need only diabdat.mpq (and hellfire mpq?), so that way can fix issues with 'cached' files from Bnet from game root or AppData folders.
Issue come with GOG releases and new Blizzard Global server. They are no back- or forward- compatible, and broke functionality for other (differ exe files). The wrong exe (or start old Updater) and you must reinstall game. Although two separate game folders and manullly edited Bnet gateways to list 5 servers (4+1 new) make possible to use old and new. Just careful with exe files (you need exe from 1.09b patch to connect to old servers, gog one are not equal to 1.09b). One miss click on exe and both installments need to be reinstalled.
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Do not start the Updater.
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If install DevuilitionX right into GOG folder and run Updater after - get some files downgraded - connect to mod server overwrite it back, right? Like old Bnet do before Global server (which one are kinda broken in that and instead of overwrite files return errors). It is don't care about existed SP or any else "minor" files?