Describe:
1) Create a composition in Photoshop
2) Create grid lines (in this example 1 vertical 1 horizontal so they intersect in the middle-bottom side of the composition)
3) Export composition using the provided Spine script (PhotoshopToSpine v7.31)
4) Open Spine > Import Data > WhateverYourFileIsCalled.jsonResult: The opened composition has the root bone aligned to the top-left instead of middle-bottom.
Spine version: 4.1.23 Pro
Compositions imported from PS to Spine does not respect PS Gridlines
You need to set the Photoshop origin. It is common to use guides (I believe guides are what you are calling gridlines) and then set the origin to the guides. That is because Photoshop sometimes forgets the origin location. To fix it you can just set the origin to where the guides are.