The Iron take on Ronnie Jepson's side at Priestfield on Saturday and Mulligan has impressed against United in the past when performing on loan in a Rushden and Diamonds shirt.
The striker, who is on loan at the Gills from Sheffield United, was stretchered off in Saturday's defeat at Doncaster Rovers.
"Our physio Simon Webster said he couldn't feel his ankle, and he felt a click, so we'll wait for the doctor's diagnosis," boss Jepson said.
"We are just waiting for the phone call."