The England midfielder's cousin Jon-Paul Gilhooley was the youngest of the 96 supporters to die in the 1989 tragedy.
"I think the timing's right and having spoken to the club I've decided to do it now," Gerrard told the Liverpool Echo ahead of the 222nd Merseyside derby at Anfield.
"It had been in the media a lot so I didn't think it was needed where I had to step in and keep it on everyone's lips and in everyone's minds.
"But now it's gone a little bit quiet over the last couple of weeks, I think the timing's really good to get it back out there - sending another message out about how much we really want this justice."
HFSGroup chairman Margaret Aspinall said Gerrard's donation would make a huge difference.
"It's a wonderful gesture by Steven and we cannot thank him enough," she said.
"It's a lot of money he has donated and it will help all the families a great deal with our work.
"It's another sign of how good the people of Merseyside are - all they have done for us and continue to do.
"The club itself has been a great help to the Hillsborough Family Support Group and we've had a lot of support from the players as well.
"This money will help with the running costs of our office and the travel expenses of the meetings and talks we attend up and down the country."