The 33-year-old ex-Torquay centre-back, brought in after injuries to a number of Rovers defenders, is available for the visit of one of his former clubs Bradford on Saturday.
Pirates manager Mark McGhee told the club's official website: "We find ourselves with a catalogue of injuries to centre halves this season. First it was Garry Kenneth, then Cian Bolger and now Tom Parkes, and we felt that we couldn't keep on improvising.
"Guy is a very experienced centre-half at this level. He is a straight-forward, uncompromising player, who competes well, and I think he will do a good job for us."
Rovers host Bradford with two points separating them from rock-bottom Barnet at the foot of League Two.