For clear glass, there usually isn’t a one-click perfect solution. Fast tools can help, but the most realistic result usually comes from building the transparency manually.

A workable approach is:

1. Keep the original layer untouched.
2. Add a temporary colored layer underneath so you can judge the cutout.
3. Duplicate the bottle image and create a layer mask.
4. Use a copy of the image plus a solid fill close to the background color to help generate the mask/highlight information.
5. Refine the mask manually so fully empty background is removed, while glass edges, reflections, and brighter highlight areas stay partially visible.

In GIMP, Color to Alpha is specifically designed for this kind of task: it converts a chosen color (such as near-white) into transparency, including partial transparency for nearly matching tones. It’s not perfect, but it can get you much closer than simple tolerance-based erasing.

In general, automated tools alone tend to produce unrealistic glass cutouts. The best-quality result is usually a combination of: removing obvious background cleanly, preserving highlights, and hand-refining the mask where the bottle is transparent.