I recently saw the phrase “persona non gratis”, which confused me. I know the phrase “persona non grata”, but have never before seen “persona non gratis”. Thought is was a mistake, so I googled it, just in case, and found a number of references to uses of “persona non gratis”. I still think it’s a misreading of “persona non grata”, but am I right? You can’t really use “persona non gratis” can you??
Persona non gratis would mean don’t let him in free, I think. 
On my limited knowledge of Latin, “grata” is an adjective modifying “persona”, so they have to match. So “grata” is the feminine form of “gratus”, meaning welcome. The English word “gratis” comes from Latin, too, but it’s a different word.
Persona non grata is correct, and as mentioned, the final “a” of grata is a necessary match to the final “a” of persona.
Persona non gratis is nonsensical, and if you found many examples of it, you found many examples of ignorance (no big surprise). No doubt it’s a conflation of persona non grata and gratis. Gratis is okay by itself, but its meaning different from that of grata in the phrase.
Thank you. I was quite sure it was wrong, but you never know, I’ve been known to make mistakes and mis-remember things 
I know what ‘gratis’ means, so it was rather surprising to see it in this context.