How can I numb my gash before stitching it shut?

See a doctor. A "gash" large enough to need closure should be closed by a physician, and you may need antibiotics or tetanus prophylaxis.
DIY? Bad idea. It has often been said that the doctor who tries to treat himself has a moron for a patient. The fact that you need to ask how to numb yourself before you suture your own wound indicates that you shouldn't be considering this. See a doctor, please.