How long does piroxicam gel take to leave your system? And can using it once make you feel ill and can it cause fatal side effects

Not likely. Piroxicam has a long half-life, about 30-60 hours. Side effects are the same as for other nsaids, and can include GI upset or ulcers, high blood pressure, kidney problems, or heart problems. Prolonged or excessive use could lead to severe toxicities or even death. Using it once or occasionally is extremely unlikely to be fatal, unless you have a severe allergy.