The 21-year old Uruguayan winger has now penned a four-year deal with the Saints for a fee in the region of £12million and manager Nigel Adkins is absolutely delighted to secure the services of the promising talent.
"A lot of hard work has gone in from the Football Club with a lot of negotiations behind the scenes to get this deal done," he told the club's official website.
"Great credit to everyone because we have a young, exciting talent joining us who is one of the best around.
"The Chairman has gone and backed this deal big-time with the financial outlay, and what we have got is one of the most exciting prospects in world football coming to St Mary's.
"There is a lot of ambition at the Football Club and we are all delighted that Gaston is going to come and join us."