Opoku, 26, scored 14 goals in 38 appearances for the Millers last season, and recently confirmed his decision to move on after four years.
Gladiators boss Paul Phillips said: “We’ve watched him and he’s big, strong, quick and powerful with bags of goals in him.
“He’s been a good servant to Carlton in the league below but he’s moving to Manchester and it’s too good an opportunity to miss for we need to add goals to our squad. Darryl Thomas speaks very highly of him and very highly of Matlock. We’re delighted to bring him to the club.”
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