Equus caballus gmelini should be synonymized with Equus caballus ferus (=Equus ferus ferus = Tarpan http://eol.org/pages/10512609/overview). ITIS has it as Equus ferus. Equus ferus przewalskii (E. caballus przewalskii) is usually considered a different subspecies.
In fact, the domesticated horse, Tarpan and Przewalskii's horse should be listed as E. ferus cabellus, E.f.ferus, and E.f.przewalskii respectively, (all subsp. of E. ferus), but none of EOL's classifications list them this way (see below).
The rules for the scientific naming of animal species are determined in the International Code of Zoological Nomenclature, which stipulates that the oldest available valid scientific name is used to name the species. Previously, when taxonomists considered domesticated and wild horse two subspecies of the same species, the valid scientific name was Equus caballus Linnaeus 1758, with the subspecies labeled Equus caballus caballus (domesticated horse), Equus caballus ferus Boddaert, 1785 (tarpan) and Equus caballus przewalskii Poliakov, 1881 (Przewalski's Horse). However, in 2003, the International Commission on Zoological Nomenclature decided that the scientific names of the wild species have priority over the scientific names of domesticated species, therefore mandating the use of Equus ferus for the horse, independent of the position of the domesticated horse (ICZN 2003).
^ International Commission on Zoological Nomenclature (2003). "Usage of 17 specific names based on wild species which are pre-dated by or contemporary with those based on domestic animals (Lepidoptera, Osteichthyes, Mammalia): conserved. Opinion 2027 (Case 3010).". Bull.Zool.Nomencl. 60 (1): 81–84.