ddickinson: Yes you do need Java. I just had a quick test and at the end of the installer it requires you to install Java. However, it seem that the Flash games (such as Machinarium) d̶o̶ ̶n̶o̶t̶ does require Flash to be installed in order to play. But if you do have Flash installed, your global settings can effect the game. For example, on Machinarium if you have global storage disabled in the Flash general settings then the game can not save properly (the saves disappear after the game is exited).
Edit: Actually, Machinarium does install Adobe Flash Player 11 ActiveX, but it does not mention this during the installation.
Perhaps GOG should indeed list Java among the games system requirements, they should do that for anything external that needs to be installed.
Thanks for the clarification, it's been a long time since I last played it, and I must had Java installed then, can't remember.
And yes, GOG should make it clear on the game page.
IIRC,
Lilly Looking Through is another case affected by one's global settings of Flash.