This is more like public announcement plus rant.
Gridmap has bad rep and I want to give it a chance, but it sure lives up to the rep lol.
I have tried and tried again and I thought I finally cracked exporting correct platform sizes/position from blender for gridmap and then something like this happened:

The Gridmap floor height doesn't match my character's default height so the platform is either too low to walk under or too high to jump on to. I know to fix it I need to change it cell_size Y but then the platforms I've made in Blender no long matched, I'd need to redo all my platform sizes/position again
eg. my slope mesh after changing cell_size Y:

So if anyone plan to use Gridmap, especially for multi levels map, don't forget to take your player's height and Gridmap floor level into account!