Due to the GIF copyright issues in the past, software and libraries to generate GIF files is pretty rare; most moved over to the PNG format long ago and gave up on GIF. And yes, PNG does not support the equivalent of an animated GIF. There are the MNG and APNG formats, but you don't see those very much (so not widely supported).
There are programs to generate movie files rom a series of PNG files. Suggestions I have seen, but never used, are avconvert (which comes with Macs), ffmpeg, programs from the ImageMagick package (which usually can be installed from package managers on Linux systems), and Movie Maker on Microsoft WIndows systems. The convert program in ImageMagick can be used to convert to GIF files, or it appears its montage program could make an animated GIF.
Karl