Why isnt it Guter Morgen/Tag?
If its Guter Plan because of plan being "der Plan." Then why its it Guter Morgen... its "der Morgen."
In case you've read (or at least watched) "The Hobbit"...
"Good morning!" said Bilbo, and he meant it. The sun was shining, and the grass was very green. But Gandalf looked at him from under long bushy eyebrows that stuck out further than the brim of his shady hat. "What do you mean?" he said. "Do you wish me a good morning, or mean that it is a good morning whether I want it or not; or that you feel good this morning; or that it is a morning to be good on?"
There's one problem less in German: "Guter Morgen!" would mean that "this is a good morning, and I want to alert you to the fact".
"Guten Morgen", as drvdw and karasu4 said, is short for "Ich wünsche dir/Ihnen einen guten Morgen" = "I wish you a good morning".
There's one added problem in German, however, if you're feeling gandalfy. If you went into a shop and asked for a good morning (to buy from them), you'd say, "Einen guten Morgen" (this is used as a greeting as well, but only rarely); or if Morgen/morning was a thing you can't count (like cheese or wine - so, maybe just "a slice of that good morning there" instead of a whole morning), you could even ask for "Guten Morgen".
Of course! Gandalf totally missed an obvious statement of fact! It was, after all, a good morning (because the sun was shining and the grass was very green). But wizards are very annoying like that - which is why witches enjoy giving them a good thrashing. A sound beating shocks them out of their self-indulgence and calms them down nicely.
ok ok ok. That makes sense... for now. But when I get to more indepth grammar and sentence word order and Guten Morgen still looks fishy... then I'll be back. But I'll take yall word for now.
Danke Schoen. Omg goodness you that i just now...at this very moment realized thats what Wayne Newton was singing/saying. Danke Schoen