There are a few differences.
Sport GT is the most different - 20" wheels, stiffer suspension & faster shifting DuoSelect transmission (only in sport mode - different software only available on this trim), louder exhaust (center h-pipe deleted), cross-drilled brakes, blacked out grilles, carbon fiber interior trim by default, leather wrapped steering wheel, sport gt badge.
The Sport GT with automatic transmission also gets the paddles by default (optional on all other trims with automatic)
Executive GT - packaged options - 19" ball polished wheels, chrome grilles, comfort pack front seats, rear side sunshades, wood steering wheel and parking brake lever to match wood trim, Executive GT badge. Late '06 availability, 06 model year Executive GT cars have rear tables, no longer available in 07.
All models available with either DuoSelect or Automatic transmissions. Paddles optional on Standard or Executive GT with automatic, standard on Sport GT. Engines \ gears are the same on all DuoSelect cars, but the automatic did get a new design. (There is a more in depth breakdown of this in another thread)