And Hessenthaler revealed that his number two made the sudden decision for personal and not football reasons.
"He felt that he couldn't fulfil the position for personal reasons. It was a shock. I'm obviously disappointed," Hessenthaler told BBC Radio Kent.
"He's a fantastic lad. I've known him for some time but, obviously, we've only worked together for a few weeks and in that short space of time we've got a lot of work done.
"Things were going well, but we've got to respect Mark's wishes.
"The chairman's left the door open for Mark to reconsider, which is great for me because I'd love to think that Mark would come back.
"But obviously I can't wait for too long because I need to move on and identify someone to fill that void now.
"I've had lots of options come through on my phone with people e-mailing me."
Robson joined the Gills as first team coach in August 2008, serving as Mark Stimson's number two until his sacking on 10 May, before then being named as number two to Hessenthaler.
Source: BBC Sport
Source: BBC Sport