Is this a standard term of some sort? I said "the entrance to success", which was wrong because of the article. I don't know that I've heard of this phrase in English, certainly not without an article.
I feel like it's sort of fixed in English. I agree that it's not the most common phrase to hear in Greek (at least not with that structure), but there is even a book for teaching English named like that http://www.macmillanenglish.com/products/gateway-to-success-a2-students-book-plus-webcode/ .
My guess is that, the original developers were based on the English phrase, and tried to translate it as accurately as possible in Greek, which ended up seeming unnatural.