The Spaniards are among several clubs monitoring the forward ahead of the summer window, with Premier League sides Chelsea and Newcastle United also credited with serious interest.
Gremio have set a minimum asking price of around €20 million and are determined not to let him leave below that figure, having previously turned down a €18 million offer from Shakhtar Donetsk.
Mec is a two-footed wide attacker with pace and the ability to play on either flank or centrally. He has been with Gremio since the age of nine and has made 18 senior appearances this season, having worn the number ten shirt for Brazil at the Under-17 World Cup.
His contract runs until 2027.
The Spanish clubs are said to be looking to move ahead of the official window opening — aware that Premier League competition for his signature is likely to intensify as the summer approaches.
Gremio have demonstrated in recent months that they are willing to hold their ground on price.