The situation is well summarized by this paragraph:
Several scholars such as April DeConick, Peter Nagel, A. Guillaumont, and G. Quispel, have advocated for an Aramaic origin of Thomas. Nicholas Perrin has argued for a Syriac original, linking Thomas with Tatian’s Diatesseron. There are others who are more convinced of a Greek origin. R. M. Grant and Helmut Koester have argued for a Greek original text. G. Garitte proposed a Coptic original for the Gospel. In the first half of The Composition of the Gospel of Thomas, Simon Gathercole argues convincingly that Thomas was originally written in Greek
Gathercole advances three main arguments for supporting a Greek original for Thomas. In his second chapter, Gathercole provides reasons to doubt a Semitic origin of Thomas. The first and most obvious argument is the need to eliminate Greek and Coptic explanations for the word before arguing for a Semitism. This makes sense since the few copies that we possess of Thomas are in these languages. Gathercole’s other reasons for preferring a Greek original are also convincing. These include the need to establish a linguistic base for identifying Semitisms, classifying Semitisms, and then assessing these Semitisms for the significance of original language composition. It is also difficult to assess Syriac influence in the text due to the paucity of Syriac literature within the period under consideration, the rarity of the translation of Syriac works into Greek, and the unlikelihood of a bilingual translation that dates back to the time when Thomas was originally composed. Gathercole also claims that there is also uncertainty that there were mistranslations or wooden translations made. With all of these difficulties, it is better to assume that Thomas did not have a Semitic origin. The simpler hypothesis is that Thomas was originally written in Greek, in keeping with the language of the earliest surviving fragments.
His third chapter provides a detailed consideration of particular texts within Thomas in which an Aramaic or Syriac original is proposed. Gathercole examines 77 sections, comparing the copies of Thomas with a proposed Syriac or Aramaic equivalent. In each of these examples, Gathercole finds reconstructing another language behind the current text of Thomas to be unnecessary. His discussion can be detailed at times. Some who do not have understanding of these extra biblical languages will have difficulty in following the argument. Even without access to other biblical languages, readers will still understand Gathercole’s point that the need to find another language underlying Thomas has been greatly exaggerated.

In his fourth chapter, Gathercole provides six reasons for a Greek original behind the text of Thomas. First, the material evidence that we possess from Thomas is in Greek. Second, there is a strong level of correspondence between the words in Thomas and the Greek language. Third, Thomas contains a high proportion of Greek loan words. Fourth, Thomas is regularly associated with other works written in Greek. Fifth, the association that Thomas has with other Nag Hammadi literature also further strengthens a Greek origin. Finally, Thomas shows a close similarity to other gospels that are written in Greek such as: Matthew, Mark, Luke, John, and the Gospel of the Hebrews.
Gathercole concludes the first part of his book by strongly stating “that a Greek original (Vorlage) to the Coptic Gospel of Thomas is a virtual certainty with proposals for a translation into Coptic from another language being highly speculative” (p. 125). With the rationale that he has assembled, his argumentation is convincing.