Along with VfB Stuttgart defender Clinton Mola, Gelhardt has been moved into the Under-21 side after PSV Eindhoven winger Noni Madueke and Huddersfield Town loanee Levi Colwil were forced to drop out due to injury.
The Young Lions confirmed the news on Monday afternoon, stating: "PSV Eindhoven winger Noni Madueke and Huddersfield Town loanee Levi Samuels Colwill will not be joining up with England MU21s for their forthcoming UEFA U21 EURO qualifiers against Slovenia and Andorra due to injury.
"Leeds United forward Joe Gelhardt and VfB Stuttgart defender Clinton Mola, who were initially part of Andy Edwards’ England MU20s, will now join Lee Carsley’s squad for their upcoming away fixtures."
Colwill missed Huddersfield's 0-0 draw at Luton Town on Saturday, prompting suggestions that he would need to wait a little longer to make his Under-21 bow.
Gelhardt, 19, was one of four Leeds players to have been named in the England Under-20s squad for this month's friendlies against Italy and Czech Republic but the striker will now join fellow prospect Charlie Cresswell in the Under-21 side.
The youngster came in for high praise from Under-20 coach Edwards last week who said: "He's very talented. Obviously we think highly of him. He is an infectious character, a great lad to have around on and off the pitch.
"He's been on the pathway for some time there and performed well. I watched him at Fulham a week or so ago and when he came on he got a really good reception so it seems he is highly thought of by the Leeds fans."