Are cracked windshields covered by Geico if one carries only liability?

No for your car, yes for someone you collided with if it was determined to be a covered accident that was your fault.

Your car;

Usually someone will need to have comprehensive coverage for glass claims to be covered.

Some policies (for a fee) will offer a zero deductible coverage if you have a higher comprehensive deductible.

Car you collided with;

Liability insurance usually (this isn't required in some states) contains physical injury and property damage. If you were At Fault and you collide with their car, their windshield should be covered.