Hi N.W.,
Can you move the limbs on left side of your body (with or without pain)? If not, this is a medical emergency. If you can move the limbs but have numbness and tingling on the skin, then you're likely dealing with a musculoskeletal issue which could be a pinched nerve but could be something else. If the entire left side of your body is affected, then the nerve involvement must be very high up your spine -- somewhere in the neck. Even so, it's unusual for a pinched cervical (neck) nerve to result in numbness of both the arms and the legs. Pinched nerves in the lumbar region (low back) or within the pelvis (hips) can result in numbness and pain in the legs and feet. If you're having spasming muscles in both your neck and along the entire spine, including the lower back, this would explain why you're having numbness along the entire left side of the body.
Your doctor is probably trying a short course of gabapentin as both a palliative and a diagnostic approach -- did he schedule a 7 day recheck with you? If not, give the office a call and find out what the doctor's proposed course of action is. If you're not happy with the answer, get a second opinion. Gabapentin is usually used for nerve pain, not numbness, although because it was actually developed as an anti-seizure medication, it can be helpful if spasming muscles are the cause of a "pinched nerve." Other medications that are frequently used for muscle spasms are Soma and Flexeril, but frankly, they will knock you on your a$$. Nerves move through muscle, so think of it this way: nerve transmission is like water moving through a garden hose. If the hose has holes along it's length and is open at the end, water gets to the source where it's needed at the end of the line, but also sprays out the holes and waters things along the way (like a soaker hose). If you squeeze the hose somewhere in the middle, water, that is nerve transmission, is blocked at the end of the line, but can still spray out higher up before the blockage. So, if you think of your hands are the muscles squeezing the nerve, this should give you an idea of how a muscle spasm can pinch a nerve and cause numbness below the point at which the spasm is occurring.
Seeing a chiropractor or an osteopath for a pinched nerve is a good idea -- they're training for dealing with such issues is different from standard medical training. Often, they will use ultrasound, heat, and massage to relieve the muscle spasm. If a misaligned disc is putting pressure on a nerve (rather than a muscle), spinal manipulation can relieve the pressure and thus the numbness. If your problems persist, you might ask your doctor to consider sending you to see a neurologist.