"The application is not free."
Translation:A aplicação não é grátis.
You are right. Grátis = de graça, gratuitamente. Its an adverb, which modifies verbs. When we use a noun (aplicação) we have to use an adjective (gratuito). Ex: the popcorn is free = a pipoca é gratuita (ela VEM (verb) grátis / de graça / gratuitamente, mas a PIPOCA é gratuita)
That is a problem that many learners have, just like in Spanish; "Ser vs. Estar." Being a native Spanish speaker, it just makes sense to use "é", but it is hard to explain; U'll try, though:
Ser: for a permanent state, defines what something is, etc
"Eu SOU um homem." "O cachorro É de cor café." "A cadeira É feita de madeira."
ESTAR: for more temporary states, defines how something is at the moment spoken, etc.
"Eu ESTOU perdido." "O cahorro ESTÁ velho." "A cadeira ESTÁ suja."
"É bom" vs "Está bom" "It is good (as a characteristic)" vs "It is well (as a state)
"É sujo" vs "Está sujo" "It is (inherently) dirty" vs "It is (currently) dirty"
Hope that helped.