If my unit was buffed by Discipline and got killed, but then saved by Guardian Angels, does it still keep the +2 from Discipline?

Yes, your unit keeps the +2 Might from Discipline.

According to RiftJudge FAQ #8433 and FAQ #810, when a unit is saved by Guardian Angel, it never technically "dies." Because Guardian Angel functions as a replacement effect, the event of the unit "dying" is replaced by the unit being healed, exhausted, and recalled to your base.

Because the unit never leaves the board (it moves only between board zones—from the battlefield to your base), it retains all of its current status, including:

• Attached equipment
• Buffs (including +2 Might from Discipline)
• Damage (which is healed by the Guardian Angel effect)
• Exhausted status (the unit becomes exhausted as part of the Guardian Angel resolution)

As noted in RiftJudge FAQ #810, "until end of turn" effects like Discipline are preserved because the unit is never sent to the trash and thus does not lose its active effects.