Okay, I think I figured this out, and as I suspected, there are things that you have to do explicitly. Umberto, I think it would be helpful to say these things specifically:
1. In order for refueling to work in the PMDG, on the FS Actions > Fuel page, you MUST adjust the LEVEL percentage line select (RSK2). When you increase the LEVEL percentage (NOT THE TOTAL LBS...), it will start slowly loading the fuel.
2. The fuel in the PMDG WILL autoload the fuel still, but GSX will SIMULATE the fuel loading from the truck...not actually load it.
Doing #1 (like in the video he posted a few posts back), it started to SIMULATE slow loading.
If you want real fuel loading, you're probably better of using PMDG's native fuel truck.
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Umberto...piece of feedback here in case it's helpful.
1. I suspect most users did what I did, and that's change the TOTAL LBS value only. Most people probably don't know what % of fuel gets them the amount they're supposed to have per OFP, so it's a big gamble there for folks to naturally use that unless you want to sit and do math, or know the volume of the fuel tank you're filling.
My suggestion is to make GSX read changes in the TOTAL LBS field also. It's not intuitive (nor explicit in any manuals) to use the LEVEL area on the fuel page.