Just to clarify.
Yixlid Jailer is in play. A spell returns Golgari Brownscale from your graveyard to your hand. Do you gain 2 life?
No, you don't. If a card is moving from a zone that's visible to everyone (e.g. in play or in a graveyard) into one that's hidden (e.g. a library or hand), we check for triggered abilities while it's visible. At that point, Golgari Brownscale has no abilities and can't trigger
Yixlid Jailer is in play, and a spell puts the top card of your library into your graveyard. It turns out to be Gaea's Blessing. Do you shuffle your graveyard into your library?
No, you don't, for the same reason: If a card moves from a hidden zone into a visible one, again we check for triggered abilities while the card is visible. Gaea's Blessing can only trigger while it's in the graveyard - but it doesn't have the ability there. Guerrilla Tactics also gets shut down this way.
Wizards did a full write up about how Jailer works here.