GIF logos and where they still turn up
GIF was the standard format for logos and buttons on the web for most of the 1990s and 2000s, so a surprising number of organisations still have their mark only as a small GIF exported from a long-gone version of Fireworks or Paint Shop Pro. It also appears in email signatures, forum avatars and the occasional animated logo. Converting one of these into a favicon is entirely possible, but GIF has two properties that set expectations: a 256 colour palette per image and a single bit of transparency per pixel.
Only the first frame is used
If the GIF is animated, the converter takes the first frame and nothing else. The browser decodes the file into an image element and the canvas draws whatever that element shows at the moment it is painted, which is frame one. Animated favicons existed briefly as a browser trick, but the ICO format has no concept of frames and this tool does not attempt to emulate it. Pick a GIF whose first frame is the finished logo, or export that frame from your editor first.
Hard-edged transparency and what it looks like small
GIF transparency is all or nothing: a pixel is either fully visible or fully invisible. Editors compensate by matting the edge pixels against an assumed background colour, usually white, so a transparent GIF logo often carries a thin light fringe. On the original site that fringe blended into a white page. In a favicon it can show as a pale outline on dark browser themes. When the canvas scales the image down to 16 or 32 pixels, those fringe pixels get averaged into the edge and can lighten the whole outline of the shape.
The output entries are PNGs, which support smooth alpha, but the converter cannot invent partial transparency that the source never had. It keeps the hard mask at sizes close to the original and produces blended edges at the smaller sizes purely as a side effect of scaling. If the fringe is objectionable, the cleanest fix is to rebuild the logo from vector artwork and use the SVG converter, or remove the background afresh with the background remover and convert the resulting PNG.
Palette limits are not a problem here
The 256 colour limit of GIF worked against photos but rarely against logos, which tend to use a handful of flat colours. Those flat colours are ideal favicon material and compress extremely well as PNG. A 200 by 200 GIF logo of 6 KB typically becomes an ICO of roughly 15 to 30 KB, with the 256 pixel entry taking most of the space because it is upscaled from a small source. Upscaling is the real weakness: a 32 pixel GIF turned into a 256 pixel entry will look soft and blocky in large icon views. It still functions, but if you have anything bigger, use it.
What the converter produces and how to use it
The result is one .ico file with five PNG entries at 16, 32, 48, 64 and 256 pixels. Save it as favicon.ico in the root of your site and, if you like, reference it explicitly with a link tag in the page head. The whole process runs in your browser; the GIF is never uploaded. Non-square GIFs are stretched to fit each square, so crop to 1:1 first with the crop tool if the logo sits on a wide or tall canvas. If you only need a single still image from the GIF rather than an icon, GIF to PNG extracts the same first frame at its original size.