Hessenthaler told BBC Radio Kent: "Obviously they've got other options and I'd expect that.
"Unfortunately, we couldn't get to them straight away because there was no manager at the club at the time."
He added: "Hopefully my influence - and that of (assistant manager) Mark Robson will mean they'll re-sign.
There will be some players we're letting go. We'll let everyone know where they stand
Andy Hessenthaler
Fuller was appointed Gills captain by former manager Mark Stimson in 2008/09, and lifted the trophy when the club won the League Two play-offs.
Richards was also part of the squad that followed that achievement with relegation from League One.
Hessenthaler said he hoped to resolve the futures of the other 11 players on the Gills' released list within days.
"I'm sure some of them have already got other clubs lined up," he said. "We'll find out the situation, but there will be some that we're letting go. We'll let everyone know where they stand."
Goalkeeper Simon Royce, defender Tom Wynter, midfielders Stuart Lewis, Mark Bentley, Adam Miller and Rashid Yussuff, and strikers Andy Pugh, Jacob Erskine, Dennis Oli, Luis Cumbers and James Walker were all out of contract after the Gills' relegation.
Source: BBC Sport
Source: BBC Sport