Palace, who are 16th in the standings, will be eager to make the most of Watford's woes and aim for a crucial win to extend their five-point advantage over 18th-placed Southampton and further stave off the threat of relegation.
Gracia said on-loan winger Gerard Deulofeu, who has been sidelined with a fractured foot, and defender Sebastian Prodl were among those hit by the virus.
"Gerard was sick this week and there were other players who have been ill, too," Gracia told reporters. "Prodl has also been ill all week.
"It is a stomach virus. They are recovered, available for the match and I am sure they are in the best condition."
Midfielder Nathaniel Chalobah (knee surgery) and defender Younes Kaboul (fractured foot), both of whom have been sidelined since September, have started training and are expected to return to action before the end of the season.
Gracia also said that Watford were yet to set a return date for midfielder Tom Cleverley, who has been out since January due to a hamstring injury.
(Reporting by Aditi Prakash in Bengaluru)
