The Malian is Gerrard's eight signing of the summer.
Coulibaly, a Mali global, could still be added to Gerrard's Europa League squad with Gers set to begin their qualifying campaign on Thursday night against Macedonian side Shkupi.
Umar Sadiq has signed a season-long loan deal with Rangers.
The 21-year-old scored two Serie A goals for the Giallorossi in the 2015-16 season, before loan spells at Bologna and Torino.
Rangers had until Friday night to submit their squad for the tie with Shkupi but Sadiq could still feature against the Macedonians if the paperwork can be completed before midnight on Wednesday.
"I sat the young lads down on day one and told them they would come over to Spain and it was down to them to mix it with the senior players and let them know you're here to challenge them", he said.More news: Soccer Coach's Training as Buddhist Monk Helps Get Thai Boys in Cave
"Rangers are a good team and I knew the name before Gerrard came in".
"I am very happy to be here".
Coulibaly has made nine appearances for Mali.
"We didn't talk that much about how the team or I would play it was all just Rangers, Rangers, and Rangers".
Sadiq will join the likes of Connor Goldson, Jamie Murphy, Allan McGregor, Jon Flanagan and Scott Arfield as new signings at Ibrox, and will give Gerrard a new look team to take into his first managerial combat with.
He smiled: "I don't like to say anything about myself but I am a hardworking player who always wants to help the team".