Generally we say that sealed jar of gel last at least 5 years, however with proper storage (cool, dry, away from direct sunlight) there is no reason for the any of our gels to expire. Some colours and glitters may separate over time, however this can be solved by simply stirring up the product with your brush or other implement.
Opened gels can be expected to last at least 2 years, however there is no official expiry date. As long as the gels are stored properly you can use them until they run out.