The 19-year-old came on as a substitute and scored in a 2-0 victory over Dunfermline last week, a display which earned him a start in Monday's Scottish Cup replay victory over Albion Rovers.
Gallagher, who spent time on loan at East Stirlingshire earlier this season, made the most of the opportunity as he laid on the first goal for Fraser Aird in the 2-0 win to secure a semi-final showdown with Dundee United.
And now Gallagher is desperate to keep his place in the team against Brechin this weekend, and repay the faith shown in him by McCoist, after admitting he came close to leaving Ibrox three years ago.
"To get my first goal and then first start was unbelievable," he said. "The confidence the manager has shown in me has been amazing.
"I was a bag of nerves five minutes before the game on Monday but I loved every minute of it. Hopefully there's more to come.
"The support I've had since then was great. My phone hasn't stopped ringing and I've been sent lots of nice, encouraging messages and tweets.
"I've even been recognised out in the street, which was a bit of a weird one. Someone came up to me and asked if I was Callum and said 'well done'.
"I told my mum and she couldn't believe it. She thought I was lying. I don't know if I'm famous yet but this sort of thing is what people dream about from a young age, so it's a real blessing.
"When I was in my last year of being part-time, I was told that the club was not sure if they wanted to keep me on or not. I'd even ended up playing in the exit trials and I was looking for teams to move to.
"But we then had the Glasgow Cup final against Celtic, where I came on for the last 20 minutes and did really well and got asked to stay on. That was the turning point for me.
"So everything this week has been a massive turnaround that I just didn't see