For additional weapon adjustments and weapon IK creations use player preview window.
To open player preview window click on 'Tools/GC Character Controller/PlayerPreviewWindow'
Assign player object to 'Player' field and weapon to adjust to 'Weapon' field.
In player preview window you can rotate, zoom and move player with assigned weapon. Also you can see aim and movement animations with assigned weapon.
To adjust weapon model click on 'Adjust weapon' checkbox.
Rotate weapon with rotation sliders and move it with middle mouse button.
There are 2 left hand IKs that can be created to fit current weapon. To create locomotion IK ( IK that will be use when walking, standing, running etc.. - none aim modes) click on 'Create IK' button ( without 'Use Aim IK' checked )
Adjust IK position by holding middle mouse button.
Adjust left hand rotations by moving rotation sliders.
Adjust how open left hand is by moving
'Open/close Fist' slider.
To create IK used in aim mode - repeat the process only with 'Use Aim IK' ticked.
For weapon setup tutorial and usage of preview window watch: