Gallop even had to change the name on the UK release of xcom to avoid copyright infringement.
said already, US release had been renamed. UK (and european) game retained name of UFO: Enemy Unknown.
And UFO TV series by Gerry Anderson, which really took role of source of the inspiration for Gallop, has nothing to do with that. It's US laws forced the rename. Since completely unrelated to both UFO TV series and UFO: EU game already was released in US under the same name. And name was offically registered (r).
As far as I know, later it was forbidden to use commonly used acronyms like UFO as a registered trademarks even in US. But UFO series became X-Com already.
and the games would be known as the UFO series instead.
At least in the Easten Europe they are exactly still known by that name. But ignorant youngling, of course, began to put all 'UFOs' to the same basket in the recent years. Thankfully to low quality Czech clones like "UFO: After...", "UFO: Extraterrestrials" etc.