Not too gruesome. Unless the beginning of the chapter pulls us into the character, really caring and rooting, then the minute the reader gets halfway through the spray of blood as the arms tore from the joint with a pop, then the book will be slammed shut. I've actually done that to a book.
Depends entirely upon the book. I've read some stories where the opening scene ends in massive bloodshed and torture. If its a dark, gritty, tale of a violent world then it probably should start with some carnage. Joe Abercrombies's "Best Served Cold" begins this way and it's an excellent read.
Your openings scenes, set a stage and serve as a hook. That hook should be consistent with what's to come, or at least a looming future threat. The greater the tension here, the better off you'll be. Some readers may close the book, others will eat it up. Write what's appropriate for your story and don't hold back. Holding back is a form of dishonesty towards your readers, an unforgivable sin in writing.