Allright, I've been thinking about the Celestial Spirits for a day and came up with an idea.
The Eclipse Gate was something from the Book of Zeref and this Gate was connected to the 12 Golden Zodiac Keys. This proves Zeref was at least aware of the Golden Keys around the time he developped the Eclipse Gate.
While thinking about the origin of the Celestial Keys, I figured that the first keys were likely created by the Sprits themselves and given to those they deemed worthy, then over the course of time they were passed from one human to the next. Since Zeref is often linked to the origin of magic, it could've been him who first learned about the existence of the Spirit Realm and its inhabitants. Perhaps he had several Golden Keys at one point, but the Spirit King did not deem him worthy for some reason, so he never got the King's Key.
However, Zeref may still have been fascinated by the Spirit King's power. The Spirit King apparently stopped time when he appeared before Loke and Lucy, so his power may be time manipulation. The power to control time must have fascinated Zeref, as he may have created Arc of Time and Last Ages, but neither of these gave him the same level of control the Spirit King had, so he opted to recreate the Spirit King's power by using the combined power of the 12 Golden Keys. This resulted in the creation of the Eclipse Gate which allowed Zeref to move through time as he pleases, should he have the Golden Keys.
Zeref may even be able to use the Gate without physically possesing the Golden Keys, as he is the one who created the Gate and thus knows all about it.
So, did Zeref base the Eclipse Gate on the Spirit King's power? Did he at one point have some Golden Keys? Will he develop an interest in Lucy? (some may want this) Would Zeref have his "private" Eclipse Gate somewhere?
So many questions! Let's see what's true and what's rubbish, but feel free to have your say about it.