I'd like to pull the bicycle out of the image, but can't seem to make any of the Gimp tutorials work.
The resolution of the image isn't enough for the job. In addition the image is spoiled by JPG artifacts. More: There are black details against black background - needs guessing. In theory you can draw a clipping mask (a hefty job) with the pen tool or erase the background literally pixel by pixel. The result will be at best poor, more probably inferior due the low resolution.
Your best bet is to take a new photo. Do the photo session targeting to the background removal. Have
- a high resolution camera, preferably RAW capable to stay away from JPGs
- strong and uniform light with no shadows and strong glossy areas!
- featureless, preferably grey background. Single color background is ok if you can keep that color out of the bike with stronger white light on the bike