The Spain Under-21 international, who has scored one goal in eight Liga appearances, has signed a deal until 2019 after the two clubs agreed an undisclosed fee.
Gil is looking forward to the challenge of playing in a new country, although he must wait for international clearance before a date for his Villa debut can be set.
"Once I knew that this club was interested in me I wanted to join," he told the club's official website.
"I am really looking forward to playing here and showing the fans what I can do.
"I can play on either side or in the middle of midfield and I like playing as part of the team, making things happen for my teammates."
The 22-year-old midfielder broke into the first team at Mestalla this season and his performances have impressed Villa boss Paul Lambert.
He hopes Gil can provide the creative spark as Villa look to improve a woeful goal record of 11 in 21 league games.
"We've been on the lookout for a creative player and he will certainly bring that extra element to the side," the Scot added.
"He's only 22 years of age but he will come here and give us something I don't think we have at the moment. He can play either role, coming in off the sides or playing in behind as well.
"He can create chances and get those chances put away. He has a lot to offer and I know we can get the best out of him."
Villa sit 13th in the table, just three points above the relegation zone.
