If I use Hostile Takeover, is the unit now considered friendly and can I kill it with Deathgrip?

Yes, if you use Hostile Takeover to take control of an enemy unit, that unit becomes a friendly unit under your control for the duration of the effect. Because it is now a friendly unit, it is a legal target for Deathgrip.

According to the rules and FAQs regarding control-changing effects:

• When you take control of a unit, it becomes your friendly unit.
• Deathgrip requires you to "Kill a friendly unit." Since the unit you took control of is now under your control, it satisfies this requirement.

As noted in the FAQ regarding Deathgrip:

"1) the friendly unit is a target, not a cost; 2) it is chosen along with the other friendly unit and killed as the spell resolves; 3) if you don’t actually kill it for any reason... you do not give the second friendly unit any Might."

Therefore, you can target the unit you took with Hostile Takeover to satisfy the "Kill a friendly unit" requirement of Deathgrip.