An ankle injury will keep Grealish out of Sunday's clash with Liverpool, while Gestede is currently doubtful for the match at Villa Park due to a hamstring problem.
The homegrown midfielder twisted his ankle during a training session earlier this week and is expected to be sidelined for around a fortnight.
“Jack went over on his ankle in a training session,” Garde told reporters. “It’s not very good and he definitely won’t be available.
“Hopefully with the break after that, Jack will be available for our next league game.”
Striker Rudy Gestede, meanwhile, is still struggling with the hamstring injury suffered after coming on as a second-half substitute in last month’s 0-0 draw at West Bromwich Albion, while Carlos Sanchez has been dealing with a similar problem since late December.
The 30-year-old Colombian started the 2-0 FA Cup third round replay win over Wycombe Wanderers three weeks ago after missing the previous five matches but was forced off at half-time due to a recurrence of the original troublesome issue.
Both he and Gestede remain doubtful to return against Jurgen Klopp’s men at Villa Park, but there could be a place on the bench for Alan Hutton after he resumed full training this week for the first time since tweaking his hamstring in last month’s 3-1 defeat at Sunderland.
“I’ve got a little hope regarding Rudy Gestede but I’m not sure I will be able to rely on him,” added Garde. “He’s not fully trained.”
Elsewhere, forward Jordan Ayew will sit out Sunday’s game while serving the second part of a three-match ban for his sending off in the 2-0 loss at West Ham United, while Libor Kozak [ankle] Adama Traore [foot] and Jordan Amavi [knee] all remain out for an extended period.