Gateway has not been truly declared as a mutant. It was hinted that his abilities could possibly be mystic in nature. Of the many teleporters, his seem to be one of the most powerful and versatile. Not only is there no strain in going through his portals, but he can teleport people not only throughout Earth, but apparently time (i.e. Jubilee communicating with her deceased parents) and possibly dimensional too.
He's definitely not a fighter, otherwise he wouldn't have taken crap from the Reaver's blackmail. He must have some telepathic means, since when the X-Men used him as a means of travel, regardless of whether they were a telepath or not, his portal always seemed to come out when they wanted it.