hi pare'ng Peachey,
"kailan ginagamit ang (salitang) GRABE?"
"grabe" is a tagalog adverb used to express excessiveness... an equivalent english term is "too".
eg. "grabe ang ulan kaninang umaga." - "it was raining too hard this morning."
as you've noticed, the term "grabe" has always been an expression of every filipino to express amazement (i.e. "wow"), regret (*** deep sigh) and even disbelief.
A: "pasado ka sa pagsusulit!" (you passed the test!)
B: "o, talaga?!, GRABE...!!! (Oh, really?!.. wow!!!)
A: "hindi ka pa makakakuha ng pasaporte." (you're not qualified to acquire a passport yet.)
B: "grabe naman 'yan... pero nakumpleto ko naman lahat ng requirements, di ba?!" [(***sigh..) but I've just completed all the requirements needed, haven't I?!)
A: "pare, pumanaw na daw yung kaibigan mo..." [(brother), I've heard that you're friend has passed away.)
B: "grabe.... kakakausap lang namin nu'ng nakaraang linggo a." (what?.. but we've jus having a conversation last week...).
there could be a lot more of way in using "grabe", I believe... I'll just allow others to share their ideas.
:)