actually, gif is an acronym. Specifically not an initialism. That means that it is pronounced as a single word (like “scuba”, but unlike “fbi” or “nsa”).
The pronunciation of the acronym does not have to conform to the original pronunciation of the letters.
Examples:
the “p” in jpeg stands for the “ph” sound, but we pronounce it as a hard “p”.
The “u” in scuba stands for “underwater”. We still pronounce it as “scOOba” not “scAAba”
So why is “gif” any different? Its creator chose the soft G for the pronunciation of the acronym (not its expansion), and therefore it is the correct one, simply because there is no rule about how it should be pronounced, so the choice was his. He made it