Q: If I bring Gengar Active after dealing damage with Farfetch’d’s (HS) “Spin Turn” attack, can a Pokemon that’s Knocked Out by damage from the Spin Turn attack be placed in the Lost Zone ?
A: Yes, it can. First, all effects of the attack are processed, then Pokemon with no HP remaining are determined to be Knocked Out. So, since Gengar’s Catastrophe Poke-Body is in effect at the time when the opponent’s Pokemon is determined to be Knocked Out, the Knocked Out Pokemon can be placed in the Lost Zone.