The youngster is now in line to make his worldwide debut against Costa Rica at Hampden Park on 23 March.
Gemmill will have to do without both players for the forthcoming European Under-21 Championship qualifier against Andorra but is confident they are up to making the next step in global football.
He said: "He's shown it, he's selected some of them in his first squad since coming back in the building, he's made it clear he's putting a real interest in not just the Under 21s but the 19s and 17s".
36-year-old Carrick confirmed his impending summer retirement on Monday and recently tipped McTominay, who has started each of United's last five games in all competitions and was named in 90min's Team of the Week after his performance against Liverpool, to replace him.
"Both of them have consistently shown that they can be trusted to play so it is fantastic for everyone that they have been given the opportunity to become part of the full squad".
"The best way to improve is to be on the pitch, of course, but he's a good learner and I remember his beginnings when sometimes he was not even on the bench but I was taking him with the team as the extra player". He is a great lad, he is humble, he takes everything in which gives him a great chance of improving all the time.
Scotland Under-21 star Oli McBurnie - now on loan at Barnsley from Swansea - and Aberdeen's Scott McKenna have also been named for the first time.
"It is a real motivation for all the players to continue their progression".
The 21-year-old, born in Lancaster to a Glaswegian father, was eligible to represent England and Scotland at global level having never played for either nation's youth sides.
The former Scotland midfielder, who was "flattered" to be linked with the senior job before McLeish's appointment, believes Grant will be a good conduit between the youth and senior teams.
"It is great for that link and the knowledge within the senior staff of what is available to them". It is not a closed door.