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The Concise New Partridge Dictionary of Slang and Unconventional English (2008) says:

Gucci adjective stylish, especially cleverly so. From the high-profile fashion brand UK, 1995

Searching Google Groups, here's a 2000 post to a Straight Dope Message Board thread on "Gen-Y Slang":

CLASSIC = stylish, "Gucci," hip, awesome

And a 2001 post to a alt.military.uk thread on "Contemporary British Army slang"

scran= food, tab=ciggie, gucci=showy or flashy, i'll think of some more later

It's quite hard to search for early uses other than in slang lists. However, when searching for bicycle-related posts, I found this February 1994 post to rec.bicycles.misc titled "Cycling jerseys. Are they better than sex?":

I was kicking around the idea of getting one of those slick looking cycling jerseys with the gucci rear pocket.

That was until I saw the price tag on these thinks. Anything from $80 to $200, depending on how stylish you want to look and whose product you want to advertise.

(Someone later replied: "Well, I don't really think it's a gucci pocket... ")

Whether this is the same use or not, the slang use is widely later used for mountain bikes from 1995 onwards.

From a July 1995 post to rec.bicycles.off-road:

You also seem to have a problem with my riding position...so what if I ride with my head on the bars!!! Well mister, next summer I'm teaching at the Colorado "How to ride your Gucci Mountain Bike better" summer camp. There's going to be a whole wave of riders emerging with their heads firmly planted on their handlebars.

An August 1995 post to rec.bicycles.marketplace was titled:

Gucci parts For Sale

An 8th November 1995 post also to rec.bicycles.marketplace titled "FS: DEAN Titanium MTB frameset 19.5in. $800":

Also available: Mag21 SL TI long travel suspension fork $225
many other components available, just ask!
bike could even be sold complete with loads of
Gucci parts.

And a week later to the same group, a reply to "Lots of High-End MTB Stuff For Sale":

I am interested in many of your bike parts but need more info. [...] How did you end up with so much gucci bike junk?