gu3: There is an open PR for a new engine in scummvm that will eventually support Wetlands:
https://github.com/scummvm/scummvm/pull/3346 eli2: In the current (August 1st, 2022) release of scummvm, Wetlands is officially supported.
However, the scummvm could not find the game in the gog folder (... Wetlands\hd\WETLANDS). The newly supported Sanitarium, however, works flawlessly.
In the folder where GOG installed the game, you'll find a "Wetlands.iso" CD image file. You can mount this or simply open it with an archive tool like 7zip. The WETLANDS folder withing this iso CD image has to be copied on top of the WETLANDS folder within the "hd" folder of your game installation, so that the two WETLANDS fodler contents will me merged (ie. do not delete the existing WETLANDS folder under "hd").
Then point ScummVM to that path, as you did, (..... hd\WETLANDS) and it should add the game properly. I've just re-tested this with ScummVM 2.6.0 and it works.