The 25-year-old, who was a key member of the Reds' championship and Premier Plate winning side in 2015/16, says he turned down several big money offers from Asia to join the Primeira Liga side.
He told Fox Sports the move was near finalised as early as January and he was excited at the prospect of playing against big name clubs like Benfica and Sporting Lisbon.
“It’s a really good move for me at this stage of my career to be in a top league like this and I’m ecstatic really that it’s all locked away,” McGowan told Fox Sports.
“It was done and dusted in January really, they saw something that they liked in me and once that interest was there it was a straight forward decision to make it happen.
“It’s a high quality league and I’ll be playing against some huge clubs. On the weekend I went to watch them against Porto and there’s obviously Benfica and Sporting Lisbon, too — Champions League and Europa League clubs, so that’ll be exciting for me to play against that quality of opposition.”
McGowan, who has represented Australia at under-20 and under-23 level, has yet to earn a Socceroos cap and hopes is move to Portugal will help turn coach Ange Postecoglou's eye.
McGowan's older brother Ryan, who plays for Chinese club Guizhou Zhicheng, has featured 18 times for the Socceroos.
“I had bigger money offers from all over Asia, most notably from Korea but I wanted to give myself a really good chance of making the Socceroos and show that ambition that’s required to be in the national team," he said.
"So hopefully this move can put me in the window for that and then I can play well enough to get that chance,” he said.