그든요 is a wrong form of 거든요.
I recommend you to use 거든요 instead of 그든요.
그든요 sounds childish.
Quoted from
http://www.learnkoreanlp.com/2012/12/because-im.html
Because I'm - ~거든요
거든요 is used as a statement in which you give reasons / explanations for what you do or did.
하거든요 = Because (I) do
거든요 is a polite spoken form
거든 is a informal spoken form (however, if 거든 is used in the middle of a sentence (i.e. as a conjunction, linking two sentences), it means 'if')
For example,
벌써 왔어? = You came (back) already?
줄이 너무 길었거든. 그래서 그냥 왔어 = Because the queue was too long. So I just came (back).
줄이 너무 길거든, 그냥 와 = If the queue is too long, just come (back).
Take 다 off verbs/adjectives and add 거든요.
왔거든요 = Because (he) came
많았거든요 = Because (there) were many
배고팠거든요 = Because (I) was hungry
빠르거든요 = Because (it's) fast
맛있거든요 = Because (it's) delicious
E.g.
아침 안 먹어요? = Don't you have breakfast?
네, 어제 저녁을 너무 많이 먹었거든요. = Yeah no, (because) I ate too much for dinner last night.
Example sentences
새 노트북 샀네요? = (Hey,) you bought a new laptop
네, 예전 노트북은 너무 느렸거든요. 그래서 새 노트북 샀어요 = Yeah, (because) the laptop I had before was too slow. So I bought a new laptop.
여기 레스토랑 음식이 진짜 맛있거든. 여기서 먹을래? = (Because) this restaurant's food is really delicious. Shall we eat here?
그래, 좋지. = Yeah, I'd like to.