MTGNexus

Gangrenous Zombies
Hopeful Eidolon

do I have this right? Playing the Zombies' ability makes the bestowed Eidolon fall off the card so lifelink does not end up attached to the ability's damage? thanks

edit: SORRY, I read the ability without the sacrifice part of the cost
Yeah you are right, to activate you need to sacrifice the zombie, so once the ability resolves zombie will already be dead and eidolon is no longer attached granting lifelink

The damage is dealt simultaneously to all creatures so you gain life because zombies are still enchanted with 2 damage on them, after the SBEs are checked all creatures with lethal damage marked will die and then eidolon falls off,

That is incorrect. Whenever an ability needs information about the source (say, to see if it has Deathtouch or Lifelink) it looks at what that source looks like currently. If, as is the case here, that source is no longer in the zone it is expected to be in, it looks at the Last Known Information of that source. Which is the information about the source as it last existed on the battlefield. And, as the Zombie last existed on the battlefield, it had lifelink. So, the damage the ability deals will result in life being gained.