I usually only come across this when I dine in large groups.
If they tack on a per-determined percentage as a so-called gratuity, I'll pay it. I won't like it, as it completely negates the very premise of a gratuity, but I'll pay it.
I will not, however, pay one cent more. The service may have been stellar. Doesn't matter. If you (read: establishment) are rude enough to tell me what the gratuity will be, the that's what you'll get. Sometimes the server will skate because the service wasn't good, though that's rare, but more often the server loses out because I probably would have left more.