What resolves first, Guardian Angel or Zhonyas?

You, the unit's controller, choose which one applies first. Both Guardian Angel and Zhonya's Hourglass are replacement effects that trigger on the same event (the unit would die). Per Rule 372, when multiple replacement effects apply to the same event, the owner of the affected object decides the order.

• If you choose Guardian Angel first: Guardian Angel is killed instead, the unit is healed/exhausted/recalled to base, and Zhonya's never triggers because the unit never died.
• If you choose Zhonya's Hourglass first: the unit is recalled exhausted, Guardian Angel remains equipped to it at base, and GA never triggers because the unit never died.

In both cases, only one gear is sacrificed and the unit is saved. (FAQ #8889, Rule 372)