Based on VSK-94's Flowing Night on the Silver Bay damaged artwork. I couldn't quite get the legs just right as they would cause clipping and make the pose look awkward.
Before you use Gpose, use a skill that pulls out your weapon first!
The pose was made using the starter weapon for the Machnist.
To move the weapon, make sure you toggle the 'P' icon for the weapon node in the hand so you can reposition it. Moving the weapon just right is probably the hardest part, so it might not fit the hand right depending on what weapon you use!