There are two matters to consider.
First, with the sole exception of elevator pits, the NEC does not require GFCI protection to be either at the breaker or the first receptacle. (Elevator pits must have a GFCI receptacle).
Second, as I've often pointed out, you need to know more than the NEC. In this instance, the running of power to the shed may require you to get a permit for the shed.
As for the shed being "portable," that is something to consider in your design. I have seen too many sheds merilly cartwheeling across the freeway on windy days! You might consider using a "power inlet" device to connect an extension cord to the shed. Or, as another mentioned, mounting the receptacle on a post near the shed.
The NEC wording is unfortunate; I would consider GFCI protection mandatory regardless of whether the shed has a floor, or not.
As a side note, there are now receptacles that have 'night lights' built in. I would consider adding one of these, just to indicate whether the power is 'on.' I also might consider one of those twist-knob timers to help prevent accidentally leaving the power 'on.'
