The Vilna Gaon adds that the document could have been called differently, for there are many combinations of letters that add up to twelve, such as a “ches” and a “daled.” However, what is unique about the “gimmel” and the “tes” is that you will never find these two letters next to each other in the entire Torah. This is why the document which is used as a separation between the man and his wife is referred to as a get.
The Steipler asked that there are other combinations of letters, such as a “gimmel” and a “kuf,” or a “zayin” and a “tzadi,” or a “samech” and a “tzadi” that are also never found next to each other!? He answers that the “gimmel” and the “tes” are the first of such combinations.
It can also be said that they wanted a name that would accomplish two things; one, that its numerical value is twelve, and secondly, that the two letters are never found next to each other. The “gimmel” and the “tes” are the only two letters that have both components.