Traditionally, implementing a new index access method meant a lot of difficult work. It was necessary to understand the inner workings of the database, such as the lock manager and Write-Ahead Log. The GiST interface has a high level of abstraction, requiring the access method implementor to only implement the semantics of the data type being accessed. The GiST layer itself takes care of concurrency, logging and searching the tree structure.
This extensibility should not be confused with the extensibility of the other standard search trees in terms of the data they can handle. For example, PostgreSQL supports extensible B+-trees and R-trees. That means that you can use PostgreSQL to build a B+-tree or R-tree over any data type you want. But B+-trees only support range predicates (<, =, >), and R-trees only support n-D range queries (contains, contained, equals).
So if you index, say, an image collection with a PostgreSQL B+-tree, you can only issue queries such as "is imagex equal to imagey", "is imagex less than imagey" and "is imagex greater than imagey"? Depending on how you define "equals", "less than" and "greater than" in this context, this could be useful. However, by using a GiST based index, you could create ways to ask domain-specific questions, perhaps "find all images of horses" or "find all over-exposed images".
All it takes to get a GiST access method up and running is to implement seven user-defined methods, which define the behavior of keys in the tree. Of course these methods have to be pretty fancy to support fancy queries, but for all the standard queries (B+-trees, R-trees, etc.) they're relatively straightforward. In short, GiST combines extensibility along with generality, code reuse, and a clean interface.