What a GIF file actually is
GIF dates to 1987 and stores at most 256 colours from a palette, compressed losslessly with LZW. Transparency is a single on/off flag on one palette entry rather than a real alpha channel, which is why GIF edges against a coloured background look jagged.
GIFs come from the reaction-image corner of the internet, from old web assets, and from screen recorders that default to it. Anything animated and low-colour that has been passed around for years is probably still a GIF.
What you get from JPG
Saving as JPEG re-encodes the image with lossy DCT compression at a quality level you choose. Around 80% is the usual sweet spot: files shrink dramatically and the difference is invisible at normal viewing sizes.
Nothing in mainstream computing fails to open a JPEG. Thirty-year-old software, print shops, hospital systems, government upload forms, every phone and every browser — if a system takes images at all, it takes JPG.
What changes when you convert GIF to JPG
This is where the file size actually drops. A GIF stores its image without discarding anything; JPEG works in 8×8 blocks and throws away the fine detail inside each one, then usually halves the colour resolution as well through chroma subsampling. Photographs take that treatment invisibly. Flat colour, text and sharp edges do not — they are where the block boundaries and coloured fringing show up, so keep the quality high if the image contains any.
Transparency is the thing to watch. GIF can store transparent areas and JPG cannot, so anything see-through is composited onto a white background during conversion. A logo with soft edges will show a white fringe when placed on a coloured background afterwards. If the transparency matters, convert to PNG or WebP instead.
Only the first frame survives. GIF is the one source here that can be animated, and JPG output is a still image, so an animated GIF converts to its opening frame. The upside for static GIFs is real: escaping the 256-colour palette removes the banding, and the file usually gets smaller at the same time.