The former Celtic player has appeared in the Champions League and Europa League but Sunday's Scottish Communities League Cup clash with Hearts is a first for the 25-year-old.
McGowan is aware it could also be his last - and he wants to make sure he has no regrets.
"Speaking to some of the boys, it's their first cup final," he said.
"It's my first cup final. Some players don't even get to play in a cup final.
"You just need to make the most of it. Obviously some of the players who played in the last cup final are still here.
"That was a hard time for them when they got beat off Rangers, it was a close game and Rangers went down two men.
"That still hurts some of the players and they want to make amends."
McGowan classes his first start for Celtic as his biggest test of character so far.
But the creative midfielder came through that with flying colours - scoring a last-minute equaliser as Celtic came back from three goals down to draw 3-3 in the Europa League.
"That was nerve-wracking," he said. "That was against Rapid Vienna over there. We were 3-0 down after 10 minutes.
"I think there will be nerves, it's a cup final. We know how we are going to approach the game, it's just whether we do it on the day or not.
"If everybody turns up, I am quietly confident we can win the cup."
McGowan and St Mirren showed they could handle the big occasion when they beat Celtic 3-2 in the semi-final, taking the game to their highly-fancied opponents from the start and bouncing back after losing an equaliser in the final minute of the first half.
McGowan said: "We just know it's one game, it's cup football and anything can happen. Hearts will be saying the same thing as well.
"They will think it's a great chance for them to win a cup as well even though the troubles they have been through. It would be a great time for them to win a cup.
"But we will do our utmost to stop them and hopefully we can bring the cup home to Paisley."