I had to check to make sure, but both "GemCraft" and "GemCraft - Chapter 0" have no possibility to submit your score to the highscore cloud. Only the "GemCraft - Labyrinth" game has this possibility out of the three.
As for submitting your score for other games, this is different for each game, provided there is a highscore involved in that particular game. When playing a game, look for a "Submit Score" button in the game and the 'HighScore Cloud' will open in a window on top of the game asking you if you want to submit your score. (Note: Some games require a renewed login for the cloud.)
