Hare, who started his second spell as head of youth in the summer, was taken ill on Tuesday evening after returning from a scouting trip.
Speaking to the club's official website, Gills boss Andy Hessenthaler said: "Unfortunately Darren has suffered a heart attack. I found out in the early hours of Wednesday morning after getting back from Stevenage.
"He has had an operation and is stable at the moment but he has had a tough few days."
A statement from the club said: "The thoughts and best wishes of everyone at the club are with Darren and his family."
Source: PA
Source: PA