Most anglophones would use a hard 'G', especially as the 'G' initial comes from the word 'Graphic' which itself uses the hard 'G', and get 'Giff'.

Most non-anglophones using the Roman alphabet would naturally use a soft 'G' before an 'I' and get 'Jiff'.

As far as I'm concerned, 'Giff' is what I myself use, but it somebody else wants to use 'Jiff', it's not going to muss my hair.

** The same arguments apply to the prefix 'giga-'. Is it 'gigga' or is it 'jigga'? Seeing that it comes from Greek, how do the Greeks pronounce it? That would be the 'correct way'. Maybe Doc Brown in 'Back to the Future' with his 'jiggawatts' was correct all along.