The 27-year-old had joined Luton on loan towards the end of last season after being suspended for disciplinary reasons under the tenure of ex-boss Dave Hockaday and had his three-year deal cancelled in February.
Mangan, who never featured for the Hatters during his time at Kenilworth Road, took his case to the Football Conference, who then overturned his dismissal.
Rovers boss Ady Pennock told BBC Gloucestershire: “He’s still under contract and he’s here to get fit.
“I knew what had gone on but I never got involved in it and I’m still not going to get involved in it.
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“He’s come in and he wants to get fit.
“We’ve given him a fitness programme and we’ll go from there.”
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