The 22-year-old striker, who has had loan spells at Hamilton and Morton, made his first SPL start in his fourth appearance this season and was voted sponsor's man-of-the-match in a comfortable victory.
Josh Thompson scored his first goal for the Hoops to give them a half-time advantage before Robbie Keane took his goal tally to nine in nine games with a penalty after the break.
Substitute Georgios Samaras, on for Marc-Antoine Fortune, completed the scoring with three minutes remaining as the Parkhead men moved 10 points behind SPL leaders Rangers, albeit having now played a game more.
However, it was McGowan who caught the eye playing behind main striker Keane and the Celtic manager said: "In my mind he has cemented his place in the first-team squad so let's hope he can push on.
"His overall performance pleased me. He is a young guy who deserves an opportunity.
"Every day in training, you see he has a great enthusiasm for the game and he is good off both feet so I'm delighted for him."
With his contract ending in the summer, McGowan knows he has picked the perfect time to come to the fore.
He said: "That's a big boost to hear what the manager said. I only found out on Friday that I was starting. I was obviously a bit nervous but I have been playing well with the reserves and training a lot with the first team.
"I've waited a long time for it. I was starting to become disillusioned.
"In the last month or two I have been there or thereabouts.
"But last week the manager told me that I would be training with the first team all the time.
"My contract is up at the end of the season so hopefully things can turn for the better.
"There was some talk a couple of weeks ago (about a new contract) but nothing yet. I will have to keep going."
McGowan added: "You can't ask for any more than learning from players like Robbie Keane.
"I like to play just behind the main striker but I'm happy to play anywhere.
"I would like to have capped it off with a goal but I'm just happy that the team won. The result matters more."
Celtic could close the gap behind Rangers to seven points with a win against St Mirren in midweek but Mowbray refused to get too excited.
"We just have to win the next game," the former Hibernian and West Brom boss said.
"I don't think we are chipping away at anything.
"Rangers have shown fantastic consistency this season, they have only lost one game I think.
"From our perspective we just have to win football matches and try to win at St Mirren in midweek."
St Johnstone manager Derek McInnes' recent luck with injuries was summed up when striker Kenny Deuchar limped off to be replaced by Steven Reynolds with only 15 minutes gone, seconds before Thompson scored the opener.
"I've never known injuries like it," McInnes said.
"We were already four strikers down and then Kenny went off injured.
"Steven Milne felt his hamstring and we had to bring him off at 1-0 and Murray Davidson limped through the last 25 minutes so we are really up against it.
"We had players patched up and some who haven't played for a while so it was tough but in reflection the scoreline was probably fair.
"This was an important game but the game against Falkirk on Tuesday was always the bigger game."