The Brazilian moves to Swansea on-loan to the end of the season.
"He is really good one-v-one. He has creativity - a lot of Brazilian swag, if you want to say it like that," said Swansea head coach Vitor Matos.
"He can play on the left, he can play on the right. Right-footed, but really good with both in terms of first touch and dribble, so we're really happy that we can have him.
"Now it is about getting the best out of him.
"The Championship is something that is different for him. He knows that as well. He has big talent, big potential, and we want to help him reach the level he wants.
"Then of course in the end of the season he'll return (to Brentford). We hope that is a perfect marriage for everyone and we can see all the best of Gustavo in this short time."