They face leaders Chesterfield on Saturday knowing they need to get a better result than either Torquay or Stevenage if they are to qualify.
"We can do it, I really believe we can," Hessenthaler told BBC Radio Kent.
He added: "I'm sure even our own people think we've got a mountain to climb and we have - but a point might even be enough.
Last year they had enough negativity by going down - we wanted to bounce back, we still can do that - it's not unthinkable
Andy Hessenthaler
"Chesterfield have got something to play for and so have we so it's going to be an interesting afternoon.
"We could get a point and the other two lose and we could still get in there."
The Gills have won just five of their last 12 games, but only failed to pick up at least a point in two games during the same period.
"We've certainly got to perform better if we're thinking about getting anywhere near the play-offs," Hessenthaler continued.
"We've got to be positive - there's no point being down about things.
"Unfortunately last year they had enough negativity by going down - we wanted to bounce back, we still can do that - it's not unthinkable.
"The pressure's on Chesterfield a bit and maybe we can exploit that. We're not out of it - it's got to be a positive week, and try and get the boys bounce back at the weekend."
Source: BBC Sport
Source: BBC Sport