agreed, this seems to be indeed an ambient occlusion problem, disabling it for the game fixes the see-through terrain glitches.
Sad that the game doesn´t support this feature as it looks really nice if it works.
Btw: for those of you that don´t want to reenable AO every time you close gw and have nvidia gpus, google for “Nvidia ispector” wich is in my opinion the best tool to manage game profiles and tweak settings for each program individually