Aaron Hickey will not make his Scotland Under-21 debut against Croatia after pulling out of the squad injured.
Hickey, 18, did not feature in Bologna's Sunday loss to Napoli and joins Kyle Magennis, Kai Kennedy and Harrison Ashby in withdrawing.
Celtic's Stephen Welsh and Lewis Fiorini - on loan at NAC Breda from Manchester City - have been called up.
The Scots, who end their qualification group away to Greece on Tuesday, can go top if they beat Croatia.
Defender Welsh has played three times for Celtic in recent weeks, including the matches against Rangers, AC Milan and Lille.
Midfielder Fiorini has featured six times this time for Dutch second-tier outfit NAC, scoring once.
"I take confidence from the performances," said Gemmill of his side's eight qualifiers so far. "The performances have been consistently at a high level. There's a real identity to how we play.
"But individually, the players have consistently performed at a high level and they're also showing that they're not only good technically, they're good mentally.
"Their attitude's very good, they're willing to do the work but also their tactical understanding at this level is also high."
Scotland Under-21 squad
Goalkeepers: Ross Doohan (Ross County, on loan from Celtic), Brian Kinnear (Rangers), Robby McCrorie (Livingston, on loan from Rangers)
Defenders: Daniel Harvie (MK Dons), George Johnston (Feyenoord), Lewis Mayo (Dunfermline Athletic), Tom McIntyre (Reading), Nathan Patterson (Rangers), Ryan Porteous (Hibernian), Patrick Reading (Ayr United), Stephen Welsh (Celtic)
Midfielders: Allan Campbell (Motherwell), Lewis Ferguson (Aberdeen), Lewis Fiorini (NAC Breda, on loan from Manchester City), Barry Maguire (Motherwell), Ross McCrorie (Aberdeen, on loan from Rangers), David Turnbull (Celtic).
Forwards: Fraser Hornby (Stade de Reims), Connor McLennan (Aberdeen), Glenn Middleton (Rangers).
- 6 Degrees from Eddie Hearn: Can the boys track down the boxing promoter?
- One Hit Wonders playlist: The biggest tracks that were never topped