Telling your enemies that you are sending troops is a mistake.
Why? They're sending troops to the region, not giving away operational details.
The goal is to avoid using the GIGN, using the threat of an attack to convince the terrorists to let their hostages go without bloodshed.
The GIGN doesn't have a great record of taking low value targets prisoner. If I were the terrorists I'd think I'd bitten off a bit more than I can chew.