Some years of Buick have a separate circuit that activates if there is a vapor leak in the evaporative emissions system. The most likely candidate is a missing or loose gas cap so the “check gas cap” light illuminates. However, anything that causes the vapor pressure in the tank to leak will also turn that light on and set the CEL.
As Texases submitted install a new gas cap. If the “check gas cap” light comes on again, you will have to take it to a mechanic to have the tank vapor recovery system diagnosed. Depending on how old this Buick is it could be a degraded hose or other seal. There are so many places the vapor pressure can leak from that throwing parts at the problem is not cost effective.
Hope this helps.