Smalley, who signed a three-month deal from Oldham at the start of November, struck in each half as the Spireites stretched their recent unbeaten streak to seven games and their lead at the summit of League Two to six points.
The brace takes the forward`s tally to four goals in eight league appearances, five of which have come from the bench.
Sheridan said: "I know what Deane Smalley can do as I know him from Oldham. He has got that little bit of confidence back now and you can see that in his play. He has had a few injuries which have set him back a bit but now he is getting a run of games and you can see the difference.
"We have spoken to Oldham about keeping him for longer and we are pretty confident something will be sorted."
Sheridan added: "It is very, very satisfying to come to a place like Gillingham and pick up three points."