Re: VW GTI towing capability?
How heavy is the trailer and bike combination? I'm assuming manual transmission. If so, then transmission loads/heat should not be an issue after moving assuming you aren't trying to go too fast with it. Check to see if the salesman can give you a towing capability ... maybe for the standard (non-GTI) version. That should give you an idea if it is reasonable.
For reference, I've been towing with my Ford Contour (low-torque 2.5L V6, manual transmission) for some years now. Both heavy (1500lb+) and light trailers (bike and single rail) with no issues. I have 115k miles on the original clutch and no issues related to towing. Just take it easy on the launch and drive at reasonable trailer speeds.
Durability really depends on how far, how often, and if there are any major hills in the route.