What might cause my 2018 GMC Sierra check engine light to come on?

In any vehicle, the check engine light is essentially a catch all for a number of different things that can go wrong, and Your 2018 GMC Sierra is no different. It's an indicator that You need to check diagnostic trouble codes (DTCs) with an OBD-II scan tool. Beyond that indication, it's impossible to tell You what Your issue may be without more information. Check engine light problems run the gamut from a loose gas cap, to an EVAP leak, to bad spark plugs, to a failed oxygen sensor, to one of no fewer than a dozen different sensors, and everywhere in between. You can typically search Vehicle History (or other sources) with a specific DTC, and You'll find more information on exactly what Your problem might be. If You don't have access to a scan tool, their use is typically offered free of charge at most automotive part stores.