The creator of the format, Steve Wilhite, intended it to be pronounced "jif", rhyming with "gift". This is how he pronounced it, and it's the pronunciation preferred by the creator.
However, many people pronounce it "gif", rhyming with "gift". This is because the word is a shortened version of "Graphics Interchange Format", and people tend to pronounce acronyms based on their letters.
Ultimately, there's no right or wrong answer. Both pronunciations are widely accepted, and it's up to you to choose which one you prefer.