tsearch2 's access method?

the basic structures to implement  the index is :

1.B- tree or the alike.

2.hash structures

So the question is arise:In the tsearch2 model, I thought the general access method is GiST tree, a tree this like B- tree.

 But when I saw the crc32_sz() function to hash the tsvector type, the index implementation again turns to be a hash list,

which is the right? hash structures or tree like B tree to implment the index?