I have an Ezekiel, Grand Master of Librarians, which is a dark angels HQ and I have a grey knight army. Can I use Ezekiel as my HQ?
Short answer: in the 7th edition of WH40k, you can create an X point Unbound army from any units you have, so yes. (Note that Unbound armies have some limitations when it comes to holding objectives, etc., so look it up in the rulebook.)
Longer answer: based on your comment to your question and depending on what rules your game group has on army building, you probably want to create a Battle Forged army, which you cannot create with the 1500 points of non-HQ Grey Knights and a Dark Angels HQ.
Grey Knights and Dark Angels can be taken as allies, but as per the rules for taking allies, you need to take the allies in an Allied Detachment and your main force in a Combined Arms Detachment.
For a Combined Arms Detachment you need, and can have:
- 1-2 HQ
- 2-6 Troops
- 0-3 Elites
- 0-3 Fast Attack
- 0-3 Heavy Support
- 0-1 Fortifications
- 0-1 Lord of War
So you need at least one Grey Knights HQ for your main detachment. For an Allied Detachment you need, and can have:
- 1 HQ
- 1-2 Troops
- 0-1 Elites
- 0-1 Fast Attack
- 0-1 Heavy Support
So to wield Ezekiel with your Grey Knights in an appropriate detachment you would need to take at least one Dark Angels Troop with him.
If you're just playing with your friends, you can probably agree with them that you would both play with 1500p, and you would just not have a HQ. But for more organised games, you need to get a Grey Knights HQ, but you would only need a single model to make your army follow the rules. After all, every army needs a leader!