We have a Halti and it is a blessing! Some boxers seem to have trouble getting used to it; it sits right under their eyes and takes a bit before they may trust it. Dempsey J. was willing to give it a go, though, and we have had little trouble.
I think the biggest difference between the Halti and the Gentle Leader is that GL makes a product designed specifically for short-nosed breeds that is more comfortable for them. I think you usually have to get it through their website.
It is certainly possible for Dempsey J. to slip the Halti, but less likely if you clip the leash to the Halti ring and his regular collar. Take your time and introudce the Halti gradually, and you'll have less head shaking and resistance that will cause a slip. Good luck, and let us know how it goes!