The 26 year old has impressed Pompey boss Steve Cotterill during his three months stay, scoring four goals in 12 games.
"We have signed Greg until the end of the season, which is great news," Cotterill told The News.
"He's done well for us, is flexible and has scored some goals. We are pleased to have him back.
"Obviously I was disappointed with him getting sent off against Watford. I thought it was very poor of him, he knows my feelings as regards to that.
"But we're delighted to have him back. It's a bit of a boost for us."