Actually, both “persona non grata” (latin has cases) and “gratis coffee/beer/bootloader” both make sense.

Just convert the “x is gratis” into “you’re welcome to [relevant-action-verb] x”.

As in, “The kernel is gratis” = “You’re free to [use] the Kernel” (which is basically “it’s free” in everyday english).

For “Persona non grata” it would be “(You’re a) person not welcome (to [come] here)”.

This is what it originally meant. It has nothing to do with price and everything to do with gratuity. I (a provider) am grateful to you and welcome you to use/come/see/do/whatever.

“Gratis” would be the ketchup packet at McDonalds - they’re happy you paid for a burger so they’ll give you a ketcup packet as they’re grateful you did.

If you replace yours after every use, then go for it!

If you keep a more traditional schedule of toothbrush replacement, I’d strongly suggest one brush per person per month. You might get away with one brush per 2 people per month, or one brush per person per up to 3 months, but not both.