Kulubi Gabriel is near Harar (about 75 km i think) in the middle of the Chercher Mountains, which are like a semi-highland area explaining the fertility/temperate weather of Harar as opposed to Dire Dawa, Jijiga, etc.
The pilgrimage that takes place every year on 26 July, the day it was consacrated (26 July 1892). The story I've gathered as to why it's holy is: after the conquest of Eastern Ethiopia by Menelik II, the Emperor trusted the region to Ras Makonen Woldemikael (father of Haile-Selassie). He became governor of Harar, a Muslim citadel in decline and which had only recently seen the Egyptians move out.
With the Abyssinian take-over, Christian settlers started arriving, and the inevitable tensions arose. More than a simple Christian/Muslim tension, it was actually 3-fold: the Harari urban aristocracy vs. the Oromo peasant-hood which had seen some marginal improvement under the Egyptians vs. the new-comers.
Apparently, Ras Makonnen made a vow to St. Gabriel that if peace was restored in Harar and its surrounding areas, he'd consacrate a church in his honour. All very Ethiopian! And so in 1892, i can only assume that with a semblance of social peace, he decided to follow his word. As to why he chose this location in particular, someone else will have to throw some light on it, i'm at a loss. What i heard is that it already had a small monastery before hand.
Again, as far as i've gathered, no exceptional miracle or event happened which would explain Kulubi Gabriel becoming such a focus of devotion.
The church, however, did gain a prominent role as the resting place of Ras Makonen, who was one the most enlightened leaders Ethiopia had just over a century ago. His respect stretch to a national and international level.
Later on, during the Italian occupation, it became a hot-bed of anti-Italian organization, much like Debre Libanos, and was bombed. Perhaps a reflection of Ras Makonen's influence in this devotion is that the church is considered holy and worthy of pilgrimage by Orthodox Christians and many Muslims in the area.
It's perfectly possible to visit Kulubi Gabriel, but do be warned that it's extremely crowded around 26 July and 26 december, on St. Gabriel's day. Accomodation in the surrounding area is limited, but transport on to Harar/Dire Dawa is constant.