Could the check engine light be due to the 100,000 mile mark or gas cap issue?

Yes the check engine light is probably related to the fuel cap light. It won't likely cause performance problems. The only drawback to not fixing it is that you will not be made aware of any new concerns with the engine that could lead to more costly repairs. You should consider getting it looked at...if it is a reasonable repair I would recommend you repair it to turn off all the lights. At least this will give you the ability to be made aware of new problems