Wait is "gratis" an actual, common English word? As a Spaniard I thought it was a joke kind of thing or used in extreme cases to differentiate, but that it was not really used in English (it is a common word in Spanish).

Similar to "libre".

It is extremely uncommon in British, Australian, and American English (as any search engine will show).

Its not uncommon in my experience as a native speaker. I feel like the context of gratis is more transactional than the context of free; eg. "Its a free country" makes more sense than "Its a gratis country".

Yeah, "gratis" literaly means "free of charge". If you ever see a "gratis country" let me know (;

Because that's not how gratis is used. In analogy to "persona non grata", that would be "America is an acceptable country". It may be gratuitous or gracious, however.