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."

Also, compare:

"É bom" vs "Está bom" "It is good (as a characteristic)" vs "It is well (as a state)

AND

"É sujo" vs "Está sujo" "It is (inherently) dirty" vs "It is (currently) dirty"

Hope that helped.

We say 'aplicativo' and 'app'. Equally, I would say. :)

"grátis" is not the same as free

• Grátis =.Costing nothing; gratuitous.
• Free = Not imprisoned or confined.

Aplicativo = app (Brazil)

Aplicação = app (Portugal)