When a bank loads, the GT scans through the buttons starting at the top left, across the top row, then moving down one row and across, assigning presets from the current bank to any preset buttons it finds. The first button it finds gets the first preset in the bank, the second button gets the second preset, etc.
However, you can use the Preset Index setting on preset buttons to change this order. Normally, this is set to 0, which disables this feature. Set Preset Index to 1 for the button where you want the first preset in the bank to go, preset index 2 where you want the second preset to go, etc. That will override the default order and assign presets in the order you choose.
For more details, please check out the videos I linked to.