The very short answer is that the GFDL is a Regional model that operates in a tightly confined box around the tropical cyclone, whereas the GFS is a global model that supplies the 'boundary' conditions for the GFDL run in a much lower data resolution.

For those of you that would like a little more detail, here is the technical explanation from NHC:

"2. Regional TC Dynamical Models

Regional TC dynamical models are dynamical models with domains that encompass the area of influence of a TC while obtaining their boundary conditions from a global dynamical model. Table 3 provides details on the operational characteristics and resolution of the nested TC dynamical models primarily used at NHC.

Table 3. Description of the most commonly used nested TC dynamical models at NHC.

Nested TC Dynamical Model Global Model Boundary Conditions Horizontal Grid Spacing Vertical Levels Coupled Ocean Model(s)
GFDL1 GFS 75Â° x 75Â° Outer grid ~30 km
11Â° x 11Â° Middle grid ~10 km
5Â° x 5Â° Inner grid ~5km 42 Atlantic: 3-D POM
Pacific: 1-D POM
GFDN2 NOGAPS 75Â° x 75Â° Outer grid ~30 km
11Â° x 11Â° Middle grid ~10 km
5Â° x 5Â° Inner grid ~5km 42 Atlantic: 3-D POM
Pacific: 3-D POM
HWRF3,4 GFS 75Â° x 75Â° Outer grid ~27 km
Inner grid ~ 9km 42 Atlantic: 3-D POM
Pacific: None

NWS Geophysical Fluid Dynamics Model (GFDL) Hurricane Model
The GFDL Hurricane Model is a limited-area, triply-nested grid-point model designed specifically for TC prediction. This grid configuration along with other technical specification for the GFDL can be found in Table 3. The GFDL is run for up to four TCs every six hours out to 126 hours as requested by NHC and CPHC. The high resolution of the GFDL allows it to resolve relatively small-scale features within a TC such as the eye and eyewall. Still, even the GFDL is not able to fully resolve the highly complex structure of a TC. The GFDL is coupled with a high-resolution version of the Princeton Ocean Model (POM), which allows TC-induced ocean modification, such as sea-surface temperature cooling, and partially accounts for the feedback of the modified ocean on the TC. In the Atlantic, the POM is three dimensional with 23 vertical levels. In the eastern North Pacific where ocean currents and sea surface temperature gradients are more predictable, only a one-dimensional POM is used. In the GFDL analysis, the GFS TC vortex is replaced with an axisymmetric vortex spun up in a separate model simulation. The axisymmetric vortex model utilizes TC specifications as provided by NHC forecasters.
Since the horizontal resolution of the GFDL is sufficiently high to represent some of the inner core TC structure, the GFDL model has up to now been the only purely dynamical model that can provide both skillful intensity and track forecasts (http://www.nhc.noaa.gov/verification/verify3.shtml).
While it is still used operationally, there are no plans to further develop the GFDL Hurricane Model. However, the GFDN model, which currently has resolution and physics similar to the GFDL, will continue to be improved."

Its important to note that there are three 'nested' grids. The outer grid with data at 30KM resolution, the middle grid with 10KM resolution and an inner grid with 5KM resolution.

Cheers,
ED