I don't really remember seeing any reason why, but I just filled up on Greville Road and the Gull is 25c cheaper than the Z.
Later drove past a Mobil and it was still 21c up.
There doesn't seem to be any specific promo happening, just cheap fuel which knocked almost 20% off the price of my 70L diesel fill.
Just drove past a Mobil who was 1.70 for diesel and 2.7x for 98.
So definitely particularly cheap, unsure if all Gull or just Greville Road.