Hi Ivan! You can use both since the words are synonyms. Although, the word "gratis" is much more used in the Swedish language and I believe the reason for that is because the word "fri" has other meanings as well. "Fri" means also to be independent or unlimited. So the word "gratis" is much more appropriate in the case of no price.

Fri can, as Christian points out, mean independent, unlimited or free. for example, the term "fri bar" literally means "free bar", while the term "free entrance" would be either "gratis inträde" OR "fri entré". Usually, in terms of price, gratis is used.