Not too much, the difficulty comes more from fewer problem choices. ICPC at the regional level has many questions to select from and usually has at least 2 problems that can be solved in under 5-10 minutes (we had a badass on our team with under 1 minute submissions). I rarely anticipate solving the most difficult GCJ problem, even the small input case. In the beginning rounds the first 2-3 GCJ problems are usually on par with easy-medium difficulty ICPC problems. As you get higher up in the rounds they get more challenging.
Edit: Should also point out that since you're not in a controlled environment, you have full internet and library access unlike ICPC where you have to rely on memorization and standard libraries.
Edit: Should also point out that since you're not in a controlled environment, you have full internet and library access unlike ICPC where you have to rely on memorization and standard libraries.