Xenonauts 1 was free few times on GoG iirc, at least this is how i got it. Generally X-COM2 from Firaxis is imo best game in the entire genre, especially there is tons of mods for it.

If you crave the original, Xenonauts 1 and 2 are closest you will get. And they are both great, just 2 is incomplete and have several problems, afaik devs did promised mod support at some point though.