Exterior wise, the Gt Sport has facelift bumper, different wheels (17" Classix) & on later models tinted windows & black headlights
Interior wise, there were a few differences that depended on the year
Heres a list of standard equiptment for the Sport, GT & GT Sport
Equipment
Sport (2005-2007):
ABS
Semi-Auto 'Climatic' Air conditioning with Pollen filter
RCD300 10-speaker (8-speaker 2006 onwards) CD Radio
Cloth seat trim
Driver's airbag
Electric Heated mirrors
Folding rear seats with 3 head restraints
Front electric windows
Isofix child seat anchor points
Speed sensitive PAS
Passenger's airbag
Remote locking (2 flip keys)
Side airbags
Space saver spare wheel
Traction control (ESP)
Alarm with interior protection
16" alloys (Either ‘Hockenheim’, 'Imola' or ‘Mugello’ depending on model year)
Lowered sports suspension (sits 15mm lower than standard Golfs)
Height adjustable Sports seats with Lumbar support
Full colour-coded exterior (strips, mirrors and handles)
‘Midline’ trip computer
Leather trimmed 3-spoke steering wheel
GT (2004-2006) adds:
Front fog lights
Automatic Lights, Wipers & Dimming Rear View Mirror
Rear Electric windows
16” ‘Imola’ Alloys
Dual-Zone Climate Control (2004/2005 ONLY)
GT (2007) adds:
17” ‘ClassiX’ alloys
New Grille design with GT badge
Boost gauge (170PS TSI only)
Twin tailpipes (170PS engines only)
‘Smooth Black’ decorative inserts
GT Sport (2008/2009) adds:
MP3 Compatible CD playback
Black section in centre of grille with 'GT Sport' badge
Tinted rear windows (B pillar back)
Dark headlights
Vienna Leather Heated Electric Seats (Jan 08 on)