wrong EJ.
Reaper and Airbellum don't "target". They RANDOMLY discard a card. In KCVDS, the most common way is to pick a card in the opponent's hand, but it is technically supposed to be RANDOM. The card to be discarded gets chosen when the effect RESOLVES not when it activates. So, what you have is:
1. Spirit Reaper eff activates
2. Gorz/Trag chained
chain resolves backwards:
3. Gorz/Trag summoned
4. Reaper/Airbellum discards a random card.