Hessenthaler told BBC Radio Kent: "He's under contract until it's the right deal for this football club.
"We don't want to stand in Simeon's way but he appreciates it's got to be right for both parties - but I'm sure that something will happen sooner or later."
He added: "If it doesn't, we've got a good player and I'm delighted he'll be staying.
"Hopefully he'll get us the goals that will help us get the promotion that we want."
Jackson scored 21 goals in the Gills' promotion campaign from League Two in 2008/9 and 18 as they were relegated from League One in 2009/10.
Source: BBC Sport
Source: BBC Sport