Can I play Deathgrip if I have no friendly units?

No, you cannot play Deathgrip if you have no friendly units.

According to Rule 355.10.c, the instruction "Kill a friendly unit" in Deathgrip targets that unit. Because the spell requires you to choose and kill a friendly unit as a target to resolve its effect, you must have at least one friendly unit available to be targeted when you play the spell.

This differs from cards like Cull the Weak, which do not target and therefore allow you to play them even if you have no units (as clarified in RiftJudge FAQ #8938 and #1732). Because Deathgrip specifically targets a friendly unit, you must have a legal target to play it.