The Baggies have the option to make the move permanent at the end of the campaign after agreeing a fee with the Egyptian outfit.
Gabr will link up with compatriot Ahmed Hegazi - another player who originally joined on a short-term contract - at The Hawthorns and has been handed the number 16 shirt.
Speaking to his new club's official website, the 29-year-old said: "I'm here to help the team and do my job. I'm very happy and excited to be here."
Hegazi and Gabr are expected to be used alongside each other for Egypt in this summer's World Cup.
Technical director Nick Hammond added: "He's a player that we first identified at the African Cup of Nations where he played alongside Ahmed. We've continued to follow his progress since then and saw a good opportunity to do a deal in this window.
"The structure is the same as for Ahmed - it's a well-structured deal for the club. Ali is strong, dominant in the air and he's got good pace as well. This is a great opportunity for him and we're delighted to have him here."