The 25-year-old impressed manager Phil Brown while training with the club and has completed a permanent move to Roots Hall having finally been given international clearance.
Gomis, who is able to play in Saturday's home FA Cup tie with Millwall, told the club's official website: " I'm really pleased to have finally signed for the club and I can't wait to play my first competitive game.
"I have really enjoyed the training so far and the players are all great with me. Hopefully we can have a really good season, the team have been doing really well in the last few weeks."
Brown said: "I'm delighted we've finally got it over the line because it's been a long process.
"We've had to jump through a number of hoops to get this one over the line and it's great that he can now be involved in the squad.
"His English is getting better by the day and he's settled in really well with all of the players.
"He has been a pleasure to work with so far. He always has a smile on his face and when he gets out on the pitch he really expresses himself with a tenacity about his play."