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There is no doubt that there was no commentary known as Gandhahasthi written by Samantabhadra on the Tattvartha. Shriyut P. Jugal Kishoreji has written in "Anekant" (Year 1, Page 216) that "the mention of the Gandhahasthi commentary appears in the Dhavala; however, I have found, through an investigation of the original manuscript of the Dhavala by 50 Hiralalji Nyayitirtha, that there is no mention of the term 'Gandhahasthi commentary' in the Dhavala.
The belief that Siddhasena Divakara is Gandhahasthi is based on a reference from the renowned scholar Upadhyaya Yashovijayji of the Eighteenth Century. Yashovijayji has quoted a verse relating to the opinion of Siddhasena Divakara regarding Gandhahasthi in his father's Mahavirasutra, and it currently seems that Siddhasena Divakara indeed is Gandhahasthi. However, Yashovijayji considers that reference to be tradition-based. Two pieces of evidence currently clarify this issue. First, no ancient or modern author prior to Yashovijayji has used the epithet Gandhahasthi in any instance alongside Siddhasena Divakara or any of their definitely recognized works, unless the excerpts were taken from those works. Only the aforementioned Yashovijayji has used the epithet Gandhahasthi with excerpts from Siddhasena Divakara's works. Thus, his statement lacks any ancient corroboration.