Yes you can in GIMP, but its hard. Well, it had everything similar to GIMP, and I made a pretty decent signature. But I dare you to try to make a signature like mine, it took me about 30 layers to get those effects.
Actually, GIMP is like Photoshop, and I think PS is stupid. I've tried PS, and it was cheap.....this signature took me 3 hours to make. Other than you takes less than that.