Full-back Dumbuya, 25, agreed a deal until the end of the season on Tuesday night after leaving troubled Portsmouth last week.
Now 20-year-old centre-half Essam arrives from the Gills on a two-and-a-half year contract.
Manager Richie Barker brought the duo in after Claude Davis joined Rotherham, while Kyle McFadzean is linked with a move to Bristol City.
Barker told Town's website: "We are delighted to have Connor at the club. He fits the template of players I want to work with. He is young with lots of potential and has a real appetite to learn and improve.
"Losing Claude Davis this week meant we had to strengthen this area of the team and we now have, in my opinion, four excellent central defenders."