Can I add an elbow to my kitchen gas line to improve clearance with the range?
Yes, you can add fittings like a 90° elbow, with these caveats:

Be sure it's legal for homeowners to do gas work in your jurisdiction.
Be sure the part is rated for gas.
Be sure you know how to properly seal threaded gas connections with thread compound or (gas rated) thread tape and good technique.
Be sure you test with soapy water when you're done.