Gills boss Andy Hessenthaler was delighted to have tied down Saunders on a longer deal.
"Mark has signed until the end of the season and hopefully when the end of the season comes we can sort him out a longer contract," Hessenthaler told the club's official website.
"I'm pleased for him - he'll be settled now for the rest of the season and it's good that he has got that bit of security he's been looking for."