I made an iPhone app where you type the file size you need and the GIF comes out under it
The clip is the actual app. I pick 10 MB as the limit, it estimates 3.4 MB, and the exported GIF comes out under the limit without me touching a single quality setting.
Why I built it: every time I made a GIF to send somewhere, it came out too big. Lower the quality, export again, still too big. Every tool I tried either made me run that guessing loop or stamped a watermark on the result.
The interesting part technically was avoiding the obvious approach. GIF size depends on resolution, frame rate, palette, and how well frames compress against each other, which is entirely content dependent, so you cannot predict it from settings. Instead of encoding and shrinking repeatedly, the app micro-encodes a few short samples of your actual clip, fits a small size model from those measurements, then does one real encode with a hard byte cap as a safety net.
It is free, has no ads or purchases, needs no account, and never uploads your video anywhere. iOS 26 or later.