A decimal point would miss the point, I’d say. To elaborate on what JayW had said, decimal points don’t actually help at all if you need anything that isn’t just a multiple of a half or a fifth of a second. No base is perfect, which is why I suggested the fractional notation that is used in the aPNG extension. This is the only way to have any frame duration. Not even if you just specify a framerate (1/n) would you be able to create every possible frame duration, it wouldn’t even allow for any duration longer than a single second.
And as JJHaggar has pointed out, you definitely shouldn’t drop millisecond and centisecond frame duration options entirely, you still need to support GIF89a somehow.