Here on gog it is possible for Garou, but not for KoF98UM... since the latter released here in 2018 directly with CodeMystics version. Btw why are you looking for them, if you don't mind saying? I'm in general interested in various versions myself, having options to choose is always preferable and version rollback should be part of the deal by default when you buy something imo.

Garou dotemu version is still sold on humble store as a drm-free version. Steam version is CM just like gog version, however both support rollback to dotemu release via console and/or beta channel. In the past, the extras section on gog had windows, linux and mac builds of dotemu version available as standalone downloads, but windows build got removed at some point.

Using galaxy, you can use rollback to dotemu (courtesy of CodeMystics, since gog doesn't really care much about preserving previous versions) by clicking on Garou > heading to settings (right to the Play button) > manage installation > configure > Beta channels - Add private channel > password "dotemuversion" > select "DotEmu 2017 Version" and select "Version gog-1". It should download. Also I would make sure to have "auto update games" turned OFF in galaxy settings, because that might cause problems with subsequent launches.

KoF98UM on gog wasn't available in their old dotemu release and afaik even humble store only has the current CM version up for sale. Only way I can see it getting rolled back to dotemu version is via steam, using the console... or scouring the web for someone having that version backed up from 2014-15. Unfortunately I don't have KoF98UM on steam, the only KoF I got there is '97GM, so can't really help with it further.

P.S. Don't waste your time asking gog support for assistance in making old versions available. I went through that just a couple of days ago in case of other SNK games and the end result was them linking me to store and telling me to go check some other games if I'm unsatisfied with the current versions. I see it as the usual "We don't care, now buzz-off" response, naturally in the most polite, consumer-friendly corporate way imaginable.