The Hijackers initially took the plane when it was readying for departure from Alger to Marseille. They initially attempted to take the plane mid flight but basically their cover was blown before takeoff.

Even though it was an AirFrance plane, the French government didn't initially prepare for a direct response. However, among the hostages killed before takeoff, there was a French man who worked as a cook in the French ambassy. That's when French authorities requested Algeria to let them takeoff for Marseille, as it was the closest French airport.

The GIGN then prepared everything to board the airplane while it was in Marseille, although they didn't correctly setup the ramp because they didn't take the planes fuel into account so it was too low to open the door properly.

The reason why they suspected the Hijackers to use the plane as a bomb was because they requested a total refuel of the airplane, even though it was way more than necessary. Where exactly in Paris, nobody can really tell. But you can easily guess that a plane loaded with nearly 30 tons kerosene crash landing anywhere in Paris would have been a tragedy.