I use the Inquisition fleet list for my Grey Knights fleet, there are some significant benefits to playing GK over normal marines.
1. All strike cruisers and your barge go 5d6 AAF.
2. All strike cruisers come upgraded with the second shield (making them MUCH more durable)
3. you re-roll boarding values if you don't like them.
4. you ignore the effects of 3 of the 4 marks of chaos.