Gallagher, 24 has spent the majority of last season on loan to Championship outfit Plymouth Argyle, where he managed 13 goals in 40 appearances for the Pilgrims; one of those was during the 2-2 home draw with Sheffield United.
It is believed that the Blades will try and sign the Scottish forward for £800,000 should attempts to bring in Ched Evans from Manchester City fall through.
