Explanation of a quote from The Great Gatsby

The sentence is confusing because it's hard to tell what the word it refers to, the building or the waste land.

It's hard for me to imagine how a waste land could minister (provide services) to a building, so I think it must refer to the waste land. Therefore, I think he's saying that the yellow building is like the stores on Main Street, providing services for the waste land.

contiguous to absolutely nothing means that there are no other nearby buildings. This is redundant, since he already said that this was the only building in sight; I guess this is meant for further emphasis of how barren the area is.

"Main Street is a generic phrase used to denote a primary retail street of a village, town or small city in many parts of the world." (Wikipedia: I would say it is a United States usage, but perhaps Wikipedia means by 'many parts of the world' both North and South Dakota).

So this building is the central part of - nothing very much.

In the sentence

The only building in sight was a small block of yellow brick sitting on the edge of the waste land, a sort of compact Main Street ministering to it, and contiguous to absolutely nothing.

I think that the author is simply saying that the block (or rectangular structure) of yellow brick is contiguous (next door) to nothing, and that the "compact Main Street" (that is, public road of modest size) leads up to it.

Under this reading, the "it" in "ministering to it" is the "block of yellow brick" (a commercial structure large enough to accommodate an empty and unrented shop, an all-night restaurant, and car repair shop); the street is "ministering" to the yellow building because it exists only to connect the building to the intercity roadway that (Fitzgerald points out earlier in the chapter) parallels a railway line on the edge of a "waste land" (which is, in fact, a tract of land covered with ashes—presumably a dump for incinerated garbage from New York City); and the building is contiguous to nothing because it is the only building on this spur street off the main roadway.

The further notion implied by "Main Street" here (as Tim Lymington notes in his answer) is that the three-business commercial building is effectively the business district of the ash dump community—except that there is no real community: no homes, no municipal buildings, no retail shops to serve a town or village of permanent residents; just a place where laborers at the dump can buy a meal and another place where passing motorists can get their cars repaired.

 To me it seems like the yellow brick building contrasts with the fact that it's in a "waste land". When it's compared to a main street, although the building is small, the diction leads me to believe a sort of aura that the building possesses. It seems to have a positive connotation and it almost seems hopeful, it reminds me of the saying "the light at the end of a tunnel".