The 30-year-old has been on loan at the Brazilian club since February and admits he would have rather stayed in his homeland than return to north London
The Brazil international struggled to secure a regular place in Arsene Wenger's side following his arrival from Fenerbahce in August 2011.
After making just eight Premier League appearances last season, Santos was sent out on loan to Gremio in February.
The 30-year-old concedes he wanted to remain with the Brazilian club but is now resigned to returning to north London.
"My loan deal is over," Santos told Radio Grenal. "It was six months long and now I am returning. Gremio had an agreement with Arsenal and I have to return."
"I wanted to stay. Gremio were fair regarding everything. I felt well there and I wanted to win a title before leaving.
"Unfortunately there is no deal and I had to return to Arsenal."