You should have the IK on in most animations, where a character is holding a gun with both hands. Notice that IK bones are also animated, exactly like the original animation. The original ik_gun and hands IK bones are there to make sure that
a) hands and feet always move the same like on the original animation, regardless of limb’s lenght or shoulder width of retargeted characters
b) left hand always holds the barrel, regardless of the character’s shoulder width, so different characters still hold the gun correctly
c) to aim the gun by rotating the ik_gun bone. It doesn’t matter what the gun is parented to
To get a result like in the above video, add two new sockets to hips or spine bone - those will be your IK targets for hands. Rotate and offset them, so when you use IK, the hands will go to positions like in the video (so, holding a gun). You can add many, many sockets more and control yhe hands of your character fully - for example steady the shot, really pull a lever, hold a torch in left hand etc. IK are cool 