IMO, if the receptacle is within 6 feet of the sink, no matter how you measure it, it should be GFCI protected. IF the origin of the rule as it pertains to 6 feet is specified because boom boxes, etc. have a 6 foot cord, then it is conceivable that some dullard could plug his radio into the receptacle while working under the sink. It has been my experience, that the majority of times that an individual dies from a 120VAC shock is when they cannot get away from it. I would certainly surmise that lying on one's back under the sink would not be conducive to recoiling from such a 120V contact.
Since it is under the sink and assuming the first contention above is correct, I would not allow for the use of a GFCI receptacle because it does not meet the definition of "Readily Accessible."
Article 100 Definitions.
Accessible, Readily (Readily Accessible) Capable of being reached quickly for operation, renewal, or inspectionswithout requiring those to whom ready access is requisite to actions such as to use tools, to climb over or remove obstacles, or to resort to portable ladders, and so forth.
Most cabinets under residential kitchen sinks are crowded with the disposal itself and a myriad of cleaning products, etc., that would require removing depending on the location of the receptacle. If one needs to merely open the door to access the receptacle and such access is not impeded by the above reiterated, then a GFCI receptacle would be acceptable.
Just one inspector's opinion.
People commonly use the phrase "YOLO - you only live once." I maintain that "you live every day....you only die once."
