Gratis is the plural ablative form of the first declension noun "grātia" in Latin and used as an adjective in various Romance and Germanic languages meaning "for nothing" [...] ... When the ablative form of this plural is used, it can mean "for the thanks", i. e. "for saying thank you", "in exchange of saying thank you".

What does gratis mean?

Gratis comes from the Latin word for "favor;" so in English a party favor is a small item given gratis to everyone attending a party. Gratis is used as both an adjective ("The drinks were gratis") and an adverb ("Drinks were served gratis"). But however it's used, it means "free".

What does it mean to do something gratis?

If something is done or provided gratis, it does not have to be paid for.