Do I get XP if I kill an enemy unit with Alpha Strike that has Guardian Angel equipped?

No, you do not gain XP if you "kill" an enemy unit that has Guardian Angel equipped with Alpha Strike.

Reasoning

1. Guardian Angel is a Replacement Effect: According to RiftJudge FAQ #8433, Guardian Angel functions as a replacement effect. When a unit would die (in this case, from damage dealt by Alpha Strike), the "death" event is replaced by "kill this Guardian Angel instead." The unit is then healed and recalled to your base.
2. Death is Prevented: Because the unit never actually "dies" and is not sent to the trash, the condition required for the Alpha Strike XP trigger is never met.
3. XP Trigger Requirement: Since the Guardian Angel effect prevents the unit from dying, it is not "killed" by Alpha Strike, and the XP gain instruction does not trigger for that unit.

Sequence of Play

1. Damage: Alpha Strike resolves, dealing lethal damage to the enemy unit.
2. Replacement: The Guardian Angel replacement effect triggers immediately, preventing the unit from dying. The Guardian Angel gear is sent to the trash, and the unit is healed and recalled to your base.
3. XP Check: Alpha Strike checks for any units killed by its action. Since no unit was sent to the trash (they were recalled to base instead), no XP is gained.

This is based on the mechanics of Replacement Effects (Rule 373) and the specific behavior of Guardian Angel (FAQ #8433, FAQ #9367).

Note: I could not fully verify this ruling against the retrieved rules.