Libre is "free" as in "free from allergies" (estoy libre de alergias) or "I am out of prison, I am free" (estoy fuera de prisión, soy libre), "gratis" refers only to the fact that you get something without paying a price for it, as in "this is a free book" (este libro es gratuito). I hope this helps you.

Libre y gratis pueden significar lo mismo: que no cuesta dinero. Ejemplo: entrada libre/gratis; pero la palabra libre tiene además otros significados.

Libre means free or freedom and gratis means free or no charge. I will give you some examples! Libre means something closer to “limitless”, it usually refers to freedom. However, you have to pay attention when conjugating libre with the verbs ser and estar. Estar libre is a less permanent/serious thing like when your time is not committed. ¿María, estás libre esta tarde? María, are you free this afternoon? While ser libre refers to the state of freedom. Lamentablemente, no todas las personas del mundo están libres. Sadly, not all the people in the world are free. Gratis is something that has no cost at all! ¿Te gustaría venir a una fiesta esta noche? Es gratis. Would you like to come to a party tonight? It's free. Puedes estacionar tu auto aquí, ¡es gratis! You can park your car here, it's free!

I think libre means to be free of something, unshackled and gratis is given away. Like buy one get one free.