Gnahoua joins the Shrimps from Bolton Wanderers, the Frenchman was one of seven players who were released by the Trotters last month.
"I'm looking forward to being a part of this team," said Gnahoua.
"It's a good club to be at and almost getting automatic promotion last season and then winning the play-offs shows how good the side is.
"My strengths are getting on the ball and creating chances, I'm hoping I can bring that to the team and score some goals this season.
"Hopefully we can do well in this league, I'm sure it's going to be tough but the gaffer is putting together a good team here and I'm looking forward getting started."
Robinson added: “I am really excited about getting this signing over the line.
"Arthur can play anywhere across the front line, he’s quick, has an eye for goal and has the ability to change a game in an instance”
Looks like (Born Again)Bill spoke too soon!