[gmueckl]

GIF is lossess iff the input has at most 256 different colors. That is the maximum color palette size and not sufficient for most cases.

[onion2k]

That's not strictly true. GIF can have a maximum of 256 colors per block, but different blocks can have different palettes, and each frame is not required to fill every block, so you can build up images with more than 256 colors using a series of incomplete frames. https://en.wikipedia.org/wiki/GIF#True_color